T 

1 


nun  inn  i   «i  ^iiii 


OF  THE 
UNIVERSITY 

OF 


QUESTIONS  AND  ANSWERS 

ABOUT 

ELECTRICITY. 

A    FIRST    BOOK    FOR    STUDENTS- 

THEORY  OF  ELECTRICITY  AND  MAGNETISM. 

EDITED  BY  E.  T.BUBIER,  2ND. 


AUTHORS: 

T.  O'CONNOR  SLOANE,   A.  M.,  E.  M.,  PH.  I). 
CARYL  D.  HASKINS,  M.  I.,  E.  E. 
A.  E.   WATSON. 
EDWARD  TREVERT. 


ILLUSTRATED, 


NEW  YORK : 

D.   VAN  NOSTRAND    COMPANY, 

1802. 


COPYRIGHTED    BY 

BUBIER     PUBLISHING    COMPANY, 
1892. 


Press  of  G.   H.   &  W.   A.   Nichols,   Lynn. 


PUBLISHER'S   PREFACE. 


This  little  work  has  been  prepared  especially  for  the 
young  student  or  amateur.  It  is  designed  for  a  first  book 
upon  electricity  and  magnetism  and  the  authors  are  all  well- 
known,  being  recognized  authorities  up'on  the  subject. 
The  arrangement  and  classification  of  the  questions  and 
answers  has  been  made  by  Edward  T.  Bubier,  2d.,  Editor  of 
Bubier's  Popular  Electrician.  We  trust  that  the  reader 
will  find  the  work  of  sufficient  merit  to  interest  and  instruct 
him. 

BUBIER  PUBLISHING  COMPANY. 
LYNN,  MASS.,  September  J,  1892. 


3 
M331733 


CONTENTS. 


CHAPTER  I. 

PAGE 

THEORY  OF  ELECTRICITY,  9 

CHAPTER  II. 
THEORY  OF  MAGNETISM,  25 

CHAPTER  III. 
VOLTAIC  BATTERIES,  31 

CHAPTER  IV. 
DYNAMOS  AND  MOTORS,  43 

CHAPTER  V. 
ELECTRIC  LAMPS,  58 

CHAPTER  VI. 
MISCELLANEOUS  ELECTRICAL  APPARATUS,  66 

CHAPTER  VII. 
ELECTRICAL  MEASUREMENT,  77 

CHAPTER  VIII. 
GLOSSARY  OF  ELECTRICAL  TERMS,  88 


QUESTIONS  AND  ANSWERS 

ABOUT 

ELECTRICITY. 


CHAPTER  I. 

THEORY    OF    ELECTRICITY. 

What  is  the  Theory  of  Electricity  f 

There  is  no  satisfactory  theory  of  electricity.  The 
best  attempts  at  theories  refer  it  to  the  luminiferous 
ether.  Stress  of  the  ether  represents  static  charges ; 
the  discharge  brings  about  relief  of  the  stress.  A 
continuous  discharge  through  a  conductor,  represents 
a  current.  The  passage  of  the  current  stresses  or 
strains  the  ether,  and  establishes  a  field  of  force.  But 
what  electricity  is  no  theory  pretends  to  tell  us. 
Charges  are  brought  about  by  contact  of  dissimilar 
substances,  as  in  rubbing  a  stick  of  sealing  wax  upon 
the  coat  sleeve,  or  by  induction  as  in  electric  machines. 
The  work  of  electricity,  its  attractions  and  repulsions 
are  assumed  to  be  done  by  the  intermediation  of  the 
ether.  If  it  throws  the  ether  into  waves  they  are 
identical  with  light  waves.  Unfortunately  we  have  not 


10  QUESTIONS    AND   ANSWEKS   ABOUT   ELECTRICITY. 

been  able  to  make  them  short  enough  to  really  give 
light  or  to  affect  the  eye.  A  current  of  electricity 
goes  through  a  conductor  because  the  conductor  opens 
a  path  for  it  through  the  ether.  A  current  cannot  go 
quietly  through  the  ether,  because  it  only  produces 
stress  or  waves  in  it. 
From  What  is  the  Word  "Electricity"  Derived? 

The  word  Electricity  is  derived  from  the  Greek 
word  "Electron"  or  amber.  It  was  noticed  by  the 
Greeks  (some  600  B.  C.)  that  if  amber  was  rubbed,  it 
possessed  the  property  of  attracting  to  itself  small 
bodies,  such  as  dust  or  bits  of  paper,  etc.  About  A.  D. 
1600,  Dr.  Gilbert  made  some  experiments  and  found 
that  other  substances,  such  as  diamonds,  glass,  sulpher, 
sealing-wax,  etc.,  possessed  the  same  property,  which 
he  styled  electrics,  since  which  time  the  word  electric- 
ity has  been  employed  to  denote  this  energy. 
W7iat  is  Electrical  Attraction  and  Repulsion? 

When  bodies  which  are  in  like  electrical  condition 
are  brought  near  togetheiythey  repel  each  other.  If 
in  unlike  electrical  condition  they  attract  each  other. 
A  body  charged  with  electricity  is  surrounded  with  a 
field  of  force,  to  which  a  species  of  polarity  or  direc- 
tion is  attributed.  The  attractions  and  repulsions  of 
electrified  bodies  are  attributed  to  the  actions  of  the 
fields  of  force  upon  each  other.  If  two  wires  are 
parallel,  and  currents  are  going  through  each,  they 
will  attract  each  other  if  the  currents  are  in  the 
same  direction,  and  repel  each  other  if  the  currents 
are  in  opposite  directions.  Here  again  the  fields  of 
force  are  treated  as  the  agents.  Each  wire  is  sur- 


THEORY  OF   ELECTRICITY.  11 

rounded  by  a  field  of  force.  If  the  currents  are  in 
the  same  direction,  the  two  fields  will  become  smaller 
if  the  wires  approach.  But  a  field  of  force  always 
tends  to  contract  itself.  Hence  the  wires  attract 
each  other.  On  the  other  hand  if  the  currents  are  in 
opposite  directions,  any  approach  of  the  wires  brings 
near  together  two  fields  of  opposite  polarity,  which 
cannot  coalesce  and  which  resist  the  crowding.  Hence 
the  wires  repel  each  other.  If  a  stick  of  sealing-wax 
is  rubbed  with  a  bit  of  silk,  the  two  attract  each 
other.  This  is  because  they  virtually  are  the 
opposite  ends  of  a  field  of  force,  which  tending  to 
contract,  draws  them  together  as  an  India  rubber  band 
would  do. 

What  is  Meant  by  Induction? 

A  current  is  said  to  be  induced  in  a  conductor  when 
it  is  caused  by  the  conductor  cutting  lines  of  magnetic 
force.  A  fluctuating  current  in  a  conductor  will  tend 
to  induce  a  fluctuating  current  in  another  running 
parallel  to  it.  A  static  charge  of  electricity  is  induced 
in  neighboring  bodies  by  the  presence  of  an  electrified 
body.  A  magnet  "induces"  magnetism  in  neighbor- 
ing magnetic  bodies. 

What  is  Static  or  Frictional  Electricity  f 

All  electricity  is  one,  but  differs  in  degree  and  dis- 
tribution. Static  electricity  is  electricity  in  repose, 
but  strongly  tending  to  motion.  The  stick  of  sealing 
wax  and  the  silk  with  which  it  is  rubbed  are  statically 
charged  with  opposite  electricities,  which  tend  to 
unite,  and  which  establish  a  field  of  force  throughout 
the  intervening  space.  Such  electrical  excitement  is 


12  QUESTIONS    AND   ANSWERS   ABOUT   ELECTRICITY. 

termed  static  electrification.  When  the  excitement  is 
enough,  as  in  a  Leyden  jar,  charged  to  a  high  degree, 
the  opposite  electricities  unite  with  a  spark  and  noise. 
Such  uniting  is  termed  the  static  discharge.  As  a 
general  rule  static  electricity  involves  very  high 
potentials  and  very  small  quantities  of  electricity. 
It  is  like  a  very  little  air  confined  at  enormously  high 
pressures. 
What  is  Thermo  Electricity  and  How  is  it  Produced? 

Thermo  electricity  is  electricity  produced  by 
heating  the  junctions  of  dissimilar  metals.  If  a  bar  of 
antimony  has  one  end  touched  to  a  bar  of  bismuth  and 
the  junction  is  heated,  a  potential  difference  will  be 
established  between  their  ends.  If  a  wire  is  carried 
from  the  unheated  end  of  one  bar,  to  the  correspond- 
ing end  of  the  other,  a  current  will  pass  as  long  as 
difference  of  temperature  is  maintained.  By  using  a 
great  number  of  couples  of  bars,  connected  in  series, 
a  considerable  current  can  be  produced. 
What  is  Atmospheric  Electricity  / 

Atmospheric  electricity  is  electricity  stored  up  in 
the  air.  or  charged  upon  the  surface  of  water  vesicles, 
the  little  spheres  of  water  which  make  up  clouds  and 
mists,  upon  floating  dust,  or  upon  minute  ice  particles 
or  in  the  air  itself.  The  cause  of  its  production,  and 
of  the  extraordinary  potential  it  may  rise  to,  is  not 
adequately  explained.  If  we  assume  a  given  charge 
to  be  contained  upon  the  surface  of  the  minute  water 
vesicles  alluded  to,  and  if  these  condense  into  rain 
drops,  their  surface  area  will  be  greatly  reduced  and  the 
potential  of  the  charge  will  rise  proportionately.  This 


THEORY   OF    ELECTRICITY.  13 

gives  some  basis  for  a  theory  of  the  high  potential 
which  produces  the  lightning  stroke  in  thunder 
showers.  Peltier  considered  that  the  atmospheric 
electricity  was  due  to  induction  from  the  earth. 
Another  theory  attributes  it  to  the  friction  of  the 
water  vesicles  against  minute  ice  particles  in  the  upper 
atmospheric  regions.  The  ice  particles  become  posi- 
tively charged,  and  the  water  particles  negatively. 
As  the  latter  fall  in  the  form  of  rain  they  carry  their 
charge  down  to  the  earth  leaving  the  ice  particles 
positively  charged  behind  them. 

No  theory  is  satisfactory.  As  a  matter  of  fact  the 
atmosphere  is  almost  always  positively  charged,  and 
the  earth  negatively,  within  nine  feet  elevation  from 
the  earth,  the  potential  may  rise  over  400  volts.  At- 
mospheric electricity  is  subject  to  perpetual  changes, 
and  occasionally  is  negative. 

What  is  Pyro-electricity  ? 

Electricity  manifested  by  certain  substances  when 
being  heated,  or  cooled.  Tourmaline,  a  natural 
mineral,  which  occurs  in  elongated  crystals  shows  it 
strongly.  If  heated,  the  distribution  of  electricity  is 
affected,  one  end  becomes  positively  and  the  other 
negatively  charged.  This  depends  on  the  change  of 
temperature.  If  allowed  to  cool,  the  polarity  is 
reversed  during  the  cooling  process.  If  kept  at  a 
constant  temperature  no  electrification  is  developed. 
The  end  of  the  crystal  which  is  positively  electrified 
with  rising  temperature  is  called  the  analogous  pole. 
The  other  is  the  antilogous  pole.  Several  other  min- 
erals, and  organic  bodies  are  pyro-electric.  Fluor 


14  QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

spar  is  affected  not  only  by  heat  but  by  light  from  the 
sun  or  from  a  voltaic  arc.  This  is  termed  photo- 
electricity. 

What  is  Dynamic  Electricity  ? 

Electricity  in  motion,  or  in  the  form  of  currents. 
An  electric  light  wire  conveys  a  current  of  dynamic 
electricity.  It  is  the  opposite  of  static  electricity, 
which  is  electricity  in  repose.  Telegraphy,  electric 
lighting  and  transmission  of  power,  electro-dynamic 
machines  and  in  general  the  commercial  electric  oper- 
ations are  in  the  field  of  current  or  dynamic  elec- 
tricity. Strictly  speaking,  the  static  discharge,  such 
as  that  of  the  Ley  den  jar  is  a  current,  but  it  is  not 
generally  treated  as  dynamic  electricity,  the  latter  be- 
ing restricted  to  current  phenomena  of  some  duration, 
or  produced  by  currents  which  are  adapted  to  be  of 
such  duration.  Nevertheless,  the  two  divisions  inevi- 
tably run  together,  being  only  varieties  of  the  same 
thing  and  the  divisions  of  static  and  dynamic  electri- 
city are  preserved  as  matters  of  convenience,  rather 
than  for  any  other  reason. 

What  is  Magneto  Electricity  ? 

Magneto  electricity  is  the  electricity  developed  by 
the  induction  in  a  magnetic  field  of  force.  If  a  con- 
ductor is  swept  across  such  a  field,  one  which  always 
exists  about  the  poles  of  a  magnet,  a  difference  of 
potential  will  be  created  in  its  ends,  and  if  those  ends 
are  connected  a  current  will  flow  through  it.  In  gen- 
eral terms  it  is  a  form  of  dynamic  electricity  produced 
in  a  special  way.  It  is  identical  with  every  other 
current  form. 


THEORY   OF   ELECTRICITY.  1& 

What  is  an  Electroscope  f 

It  is  an  instrument  for  detecting  whether  a  body  is 
electrified  and  whether  the  electrification  is  positive 
or  negative.  There  are  several  varieties,  the  best  and 
most  sensitive  being  the  gold  leaf  electroscope.  It 
may  be  described  as  follows  : 

Two  strips  of  gold  leaf  are  suspended  within  a 
glass  jar  or  wide-mouthed  bottle  upon  a  stiff  piece  of 
brass  wire.  The  wire  is  enclosed  within  a  glass  tube 
which  is  pushed  through  the  cork.  Both  cork  and 
tube  should  be  well  shellaced  or  paraffined.  Upon 
the  upper  ends  of  the  wire  is  a  round  flat  piece  of 
copper.  This  is  a  very  delicate  instrument  and  if  it 
is  kept  perfectly  dry  and  free  from  dust  will  indicate 
very  small  quantities  of  electricity.  This  instrument 
will  show  that  all  kinds  of  friction  will  produce  elec- 
tricity. If  a  glass  rod  be  rubbed  with  dry  silk  and 
placed  near  the  top  of  the  electroscope  the  gold 
leaves  will  be  seen  to  repel  one  another.  If  you  wish 
to  see  whether  a  body  is  electrified  negatively  or  posi- 
tively proceed  as  follows:  First  charge  the  electro- 
scope by  touching  the  metal  plate  on  top  with  a  glass 
rod  previously  rubbed  with  silk.  This  will  make  the 
leaves  diverge.  Now  if  you  approach  the  electroscope 
with  a  positive  electrified  body  the  leaves  will  tend  to 
diverge  still  further,  or  if  with  a  negatively  electrified 
body  they  will  tend  to  close. 

What  is  a  Conductor? 

A  substance  which  will  allow  the  passage  of  electric- 
ity over  it.  All  substances  will  do  this,  but  some  to 
so  small  an  extent,  that  they  are  called  insulators. 


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18  QUESTIONS   AND   ANSWEES  ABOUT   ELECTRICITY. 

to  six  revolutions  for  each  revolution  of  the  driving 
wheel,  the  plates  of  small  machines  requiring  a  more 
rapid  revolution  than  those  of  larger  ones.  In  front 
of  the  plate  A,  \  of  an  inch  from  the  glass,  are  the 
combs  Kand  H,  attached  to  a  brass  core  at  the  centre 
of  the  ebonite  disc  M ;  and  the  combs  K  and  L,  in- 
sulated by  their  attachment  to  ebonite  rods  projecting 
from  the  disc  M^  and  connected  by  brass  rods  with 
the  Leyden  jars  C  and  Z>,  and  with  the  sliding-rods 
P  and  R.  These  sliding-rods  have  ebonite  handles, 
and  terminate  in  brass  balls  at  their  inner  extrem- 
ities. 

The  plates  of  sheet  glass,  about  1  of  an  inch  thick  ; 
of  good  insulating  quality,  and  well  coated  with 
shellac.  The  stationary  plate  B  has  two  circular 
openings  called  windows,  directly  opposite  the  combs 
JTand  L\  and  on  its  rear  surface,  are  cemented  two 
paper  inductors  3Tand  -3T;  J5Textendingfrom.Zrto  L, 
and  JTfrom  F'to  K\  and  each  armed  with  a  row  of 
points  projecting  into  each  window. 

The  Topler  machine  shown  on  next  page  has  the 
same  general  construction  as  the  Holtz ;  but,  on  the 
front  surface  of  the  revolving  plate,  are  cemented  a 
number  of  small  metal  discs,  called  carriers ;  usually 
made  of  tin-foil  with  raised  brass  centres,  which,  as 
the  plate  revolves,  are  brought  into  contact  with  four 
brushes;  two  attached  to  a  stationary  plate,  and  two 
to  the  uninsulated  combs.  In  this  way  the  machine 
is  made  self-inciting,  as  already  mentioned. 

The  windows,  and  the  rows  of  points  projecting  in- 
to them,  used  in  the  Holtz  stationary  plate  are  omit- 


THEORY   OF   ELECTRICITY. 


19 


ted  from  the  stationary  plate  of  the  Topler,  and  the 
paper  inductors  are  made- longer,  and  have  small  tin- 
foil inductors  under  them,  connected  by  tin-foil  strips 


THE   TOPLER  MACHINE. 


with  each  other  and  also  with  the   two   brushes  at- 
tached to  this  plate. 

This  machine  was  constructed  by  Phillip  Atkinson, 
and  patented  April  10,  1883,  and  December  8,  1885. 


20  QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

The  principal  points  covered  by  the  patents  are  as 
follows : 

1.  The  outside  coatings  of  the  Leyden  jars  C  and 
D  are  of  sheet  brass,  nickel  plated  ;  and  are  screwed 
firmly  to  the  base,   forming  cups  into  which  the  jars 
fit  closely,  and  are  thus  held  in  a  fixed  position ;  af- 
fording a  firm  support  to  the  parts   connected  with 
them,  and  preventing  liability  to  accident  or  injury  to 
the  jars  or  plates. 

2.  The  induced  current  from  these  outside  coat- 
ings is  conveyed  down  by  the  brass  screws -which  at- 
tach them,  and  along  copper  wires  underneath,  to  the 
terminals   of   the   switch    S\    through   which,   when 
closed,  it  passes  from  one  jar  to  the  other ;  but  when 
open,  as  in  the  cut,  it  passes  by  the   brass   sockets, 
seen  on  the  edge,  which  are  also  connected  with  the 
terminals,  out  through  the  conducting  cords,  and   a 
person,   or  other  object,  connected   with  their  outer 
extremities.   As  this  induced  current  flows  simultane- 
ously with  the  direct  current  from  the  inside  coatings, 
the  switch  and  sliding-rods  place  it  completely  under 
control  of  the  operater. 

3.  The  brush  holders,  E  and  F,  are  attached  to 
the  plate  .Z?,  through  holes  near  its  edge ;  thus  giving 
a  direct  passage  to  the  electricity  from  the  carriers 
on  the  plate  A,  where  it  is  generated,  through  the 
glass,   to   the   tin-foil  inductors,  represented  by   the 
dark  shade,  and  the  paper  inductors  .T'and  X,  repre- 
sented by  the  light   shade.     By  passing  the  electric 
charge  through  the  glass^  inside  its  edge^  an  insulat- 
ing margin  is  interposed  between  the  conductors  and 


THEORY   OF   ELECTRICITY.  21 

the  edge,  thus  preventing  loss  from  leakage,  which  is 
unavoidable  when  the  brush  holders  are  attached  by 
clamps  or  ears  on  the  edge. 

4.  The  carriers  on  the  plate  A  are  of  sheet  brass, 
with   raised    centres,   and  are  nickel  plated,  making 
them  both  durable  and  ornamental.     The  hard  nickel 
surface  is  affected  by  the  action  of  the  brushes,  or  the 
electricity,  while  tin-foil  soon  becomes  defaced ;  and 
the  carrier,  being  practically  one  piece,  and  its  entire 
surface  cemented  to  the  glass,  its  raised  centre  can- 
not become  detached,  as  may  happen  when  the  centre 
is  put  on  separately  over  a  tin-foil  base. 

5.  The  combs  T^and  J^  also  JTand  X,  radiate  at 
an  angle  of  45  degrees  to  each  other,  from  the  central 
disc  7!/,  to  which  they  are  attached ;  so  that  any  pos- 
sibility of  error  in  regard  to  their  position,  or  of  dis- 
placement, is  practically  impossible. 

The  following  improvements  may  also  be  noticed : 

The  base  is  made  of  two-inch  strips,  glued  together 
lengthwise,  and  heavy  cleats  screwed  on  underneath; 
giving  all  the  advantages  of  iron  as  to  freedom  from 
warping,  with  the  insulation  and  elegant  finish  of  the 
wood.  The  driving-wheel  is  of  ebonite  and  the  iron 
casting,  on  which  it  is  mounted,  slides  in  grooves  on  an 
iron  plate,  and  is  moved  by  the  adjusting  screw  0,  to 
regulate  the  tension  of  the  belt. 

The  ebonite  insulators,  which  support  the  plate  .Z?, 
have  soft  rubber  packing,  to  ease  the  pressure  on  the 
glass. 

The  conducting  rods  of  the  Leyden  jars  pass 
through  ebonite  caps  with  cork  attached  underneath, 


22  QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

which  gives  them  a  fixed  vertical  position,  and  affords 
firm  support  to  the  sliding-rods"  and  the  combs  con- 
nected with  them  above. 
What  is  a  Condenser? 

It  is  an  apparatus  for  collecting  and  holding  electric- 
ity. It  consists  of  alternate  layers  of  conducting  sheets 
and  insulating  material,  the  conductors  being  very 
close  together,  and  the  adjacent  ones  being  charged 
with  the  opposite  kinds  of  electricity.  Their  proximi- 
ty enables  them  to  hold  a  larger  amount  of  electricity 
than  they  could  if  alone.  Condensers  are  sometimes 
called  accumulators.  A  condenser  is  the  modern  and 
practical  form  of  a  Leyden  jar,  altered  in  appearance 
and  used  for  different  purposes.  Its  construction  in- 
volves tinfoil,  but  varnished  silk  or  paper  is  used  in- 
stead of  glass.  Alternate  sheets  of  tinfoil  are  con- 
nected to  one  wire  or  binding  post,  the  rest  of  the 
sheets  to  another  post.  The  effect  is  the  same  as  if 
but  two  large  sheets  were  used,  of  an  area  equal  to  the 
sum  of  the  small  ones.  Condensers  are  used  in  the 
bases  of  large  induction  coils.  At  the  make  and  break 
contacts  on  such  coils,  destructive  sparks  are  con- 
stantly burning.  The  condenser  dissipates  these 
sparks  and  increases  the  intensity  of  the  induction  in 
the  coil.  The  amount  of  electricity  a  condenser  will 
hold  in  the  static  form  is  measurable.  This  leads  to 
valuable  practical  application.  Sometimes  an  ocean 
telegraph  cable  breaks ;  it  becomes  necessary  to  locate 
the  fault  by  measuring  the  static  capacity  of  the  cable 
and  comparing  the  results  with  a  condenser  of  known 
capacity,  the  distance  to  the  break  can  be  calculated. 


THEORY  OF   ELECTRICITY.  23 

Describe  a  Leyden  Jar  ? 

Opposite  conditions  of  electricity  attract  one  an- 
other, and  although  electricity  cannot  flow  through 
glass  it  can  act  across  it  by  induction.    For  example  : 
Placing  a  plate  of  glass  between  two  pith  balls,  one 
being  electrified  positively,  the  other  negatively,  will 
not  interfere  with  their  attracting  or  repelling  one 
another,   although  the  electric   charges   cannot   pass 
through  the  glass.     On   this   principle   was  invented 
the  Leyden  jar,  and  other  condensers.     The  Leyden 
jar  was  accidentally  discovered   by  Musschenbroek, 
and  his  pupil  Cuneus,  in  the  town  of  Leyden,  from 
which  it  derives  its  name.     It  usually  consists  of  a 
glass  jar  on  which  is  pasted  two  coatings  of  tin-foil, 
one  on  the  inside,  and  one  on  the  outside,  the  coating 
covering  the  jar,  three-fourths  of  its  length.     Electric 
connection  is  made  by  a  chain,  or  a  flexible   wire 
hanging  into  the  jar  from  a  brass  rod,  which  may  be 
supported  by  a  wooden  cover  to  the  jar,  to  which  the 
rod  is  fixed.     A  brass  nob  is  attached  to  the  top  of  the 
rod.     To  charge  the  jar,  it  is  necessary  to  hold  or  con- 
nect this  nob  to  the  prime  conductor  of  an  electrical 
machine  ;  the  outer  coating  being  either  held  in  the 
hand,  or  connected  to  the  earth  by  a  wire.     The  jar 
can  be  easily  charged  in   a  few  minutes  in  this  way, 
and  if  made  of  good  glass,  kept  dry  and  free  from  dirt, 
will  retain  its  charge  for  many  hours.     The  jar  may 
be  discharged  by  holding  it  in  one  hand  by  the  outer 
coating  and  touching  the  brass  nob  with  the  other  hand. 
The  person  so  doing  will  see  a  bright  spark  pass  be- 
tween the  nob  and  the  hand,  making  a  sharp  report, 


24  QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

and   at   the    same    time    giving    him    a    convulsive 
shock. 

A  very  simple  Ley  den  jar  can  be  made  in  the  fol- 
lowing manner,  and  was  the  original  experiment  of 
JVfusschenbroek  and  his  pupil :  Take  a  glass  bottle, 
fill  it  about  two-thirds  full  of  water,  make  a  hole 
through  the  cork  and  push  through  it  a  long  nail,  so 
that  it  hangs  -low  into  the  water,  when  the  cork  is  in 
the  bottle.  This  jar  can  be  charged  like  the  modern 
Leyden  jar,  and  in  the  same  way,  the  water  acting  as 
the  inner  coating  and  the  hand  as  the  outer.  When 
the  jar  is  charged,  it  can  be  discharged  by  holding  it 
in  one  hand  and  touching  the  top  of  the  nail  with  the 
other.  Thin  glass  has  a  greater  capacity  as  an  accu- 
mulator, than  thick  glass,  but  if  the  glass  should  be  too 
thin,  the  jar  will  be  liable  to  be  destroyed  by  the 
spark  of  a  powerful  charge  actually  piercing  it.  A 
powerful  battery  may  be  made  of  Leyden  jars  by  con- 
necting a  number  or  them  together  by  thin  inner  coat- 
ings, then  also  uniting  their  outer  coatings.  Care 
should,  however,  be  taken  in  discharging  this  battery 
of  Leyden  jars,  by  using  a  pair  of  discharging  tongs, 
as  a  shock  from  such  a  battery  might  prove  fatal. 
The  discharging  tongs  is  an  arrangement  consisting 
of  a  brass  rod  with  two  brass  knobs,  and  insulated 
from  the  hand  by  a  glass  handle. 


THEORY     OF     MAGNETISM.  25 


CHAPTER  II. 

THEORY    OP   MAGNETISM. 

What  is  the  Theory  of  Magnetism  ? 

The  most  generally  accepted  theory  is  known  as 
Ampere's  theory ;  it  holds  that  every  particle  of  a 
magnet  has  a  minute  current  of  electricity  circulating 
around  it ;  that  the  result  of  the  aggregation  of  these 
particles  with  their  currents  is  the  production  of  a 
sheet,  as  it  were,  of  current  circulating  around  the 
outside  of  the  magnet.  If  we  look  at  the  North  pole 
of  a  magnet,  the  current  is  assumed  to  go  around  it  in 
a  direction  opposite  to  that  of  the  hands  of  a  watch, 
and  the  reverse  for  the  South  pole  naturally.  No  en- 
ergy is  expended  in  keeping  up  this  current  because 
each  one  of  the  constituent  currents  is  of  such  minute 
dimensions  that  it  maintains  itself,  once  started. 

What  is  Meant  by  Magnetic  Attraction  and  Repul- 
sion f 

This  is  really  a  continuation  of  the  preceding  an- 
swer. We  have  seen  that  currents  in  similar  direc- 
tions tend  to  approach  each  other.  Bearing  in  mind 
the  direction  of  the  Ampereian  currents  it  will  be 
seen  that  if  the  North  and  South  poles  of  a  magnet  face 
each  other,  the  currents  of  each  will  be  in  the  same 
direction  ;  hence  the  North  and  South  pole  attract 
each  other.  If  two  poles  of  similar  names  face  each 


26  QUESTIONS    AND   ANSWERS    ABOUT   ELECTRICITY. 

other,  the  currents  will  be  in  opposite  directions  and 
therefore  will  repel  each  other.  Another  way  to  look 
at  it  is  to  consider  the  field  of  force  which  these  Am- 
pereian  currents  create.  If  the  North  and  South 
poles  of  two  magnets  are  brought  near  each  other  the 
field  extends  from  one  to  the  other  and  they  are 
drawn  together  by  the  natural  tendency  to  contrac- 
tion of  a  field  of  force  as  if  rubber  bands  were  stretched 
between.  If  similar  poles  are  brought  together  the 
opposite  fields  of  force  cannot  coalesce,  and  they  push 
each  other  away  just  as  two  inflated  rubber  balls  would 
if  pressed  together. 
Will  a  Magnet  Act  Across  Bodies? 

The  action  of  a  magnet  is  to  some  extent  shielded 
or  cut  off  by  any  piece  of  iron.  This  is  done  by  the 
lines  of  force  which  should  "emanate  from  its  poles 
being  diverted  or  turned  aside  owing  to  their  high 
affinity  for  metallic  iron.  No  body  other  than  iron 
and  a  few  other  substances,  acts  thus,  and  they  only 
act  by  concentrating  upon  themselves  the  attractive 
power  of  the  magnet.  It  is  not  a  genuine  cutting  off 
of  the  attraction. 
How  may  Magnetism  be  Derived  from  the  Earth  f 

By  subjecting  a  bar  of  steel  to  molecular  disturb- 
ance while  held  in  the  magnetic  meridian  or  approx- 
imately north  and  south.  The  disturbance  may  take 
the  form  of  jarring,  hammering,  twisting  or  the  like. 
It  is  as  if  the  polarity  of  the  earth  was  striving  to 
make  a  magnet  of  it  and  could  only  succeed  effectu- 
ally when  it  was  helped  on  its  way  by  the  disturbance 
of  the  molecules. 


THEORY  OF   MAGNETISM.  27 

What  is  Meant  by  Magnetic  and  Diamagnetic  Bodies  f 
A  magnetic,  paramagnetic,  or  ferromagnetic  body 
is  one  which  is  attracted  by  the  magnet,  which,  if 
suspended  in  the  neighborhood  of  a  magnet  tends  to 
stretch  from  pole  to  pole  thereof ;  which  has  a  high 
affinity  for  magnetic  lines  of  force,  and  through 
which  lines  of  force  (the  elements  of  a  field  of 
force)  pass  more  readily  than  they  do  through  air. 
A  diamagnetic  body  is  exactly  the  reverse  of  the 
above.  Suspended  in  the  neighborhood  of  a  magnet 
it  tends  to  place  its  longest  axis  across  the  line  con- 
necting the  poles,  it  has  less  affinity  for  lines  of  force 
than  has  air  and  tends  to  be  repelled  by  a  magnet. 

What  are  the  Laws  of  Magnetic  Force  ? 

Theoretically  the  laws  are  very  simple ;  magnetic 
force  varies  inversely  with  the  square  of  the  distance 
or  follows  the  law  of  inverse  square.  In  practice 
owing  to  the  different  shapes  of  magnets,  and  the  size 
of  their  poles  which  is  large  compared  to  the  distance 
through  which  they  act,  the  law  of  inverse  squares 
really  does  not  apply  and  no  general  law  can  be 
stated  as  a  special  law  will  exist  for  each  case. 

What  is  a  Natural  Magnet  ? 

The  natural  magnet  or  "lodestone"  is  an  iron  ore, 
and  frequently  occurs  in  crystals.  This  ore  is  mag- 
netic, that  is,  it  has  the  power  of  attracting  iron  and 
steel  to  itself,  it  is  known  to  minerologists  as  magnetite, 
it  also  has  the  property  of  pointing  north  and  south 
when  it  is  suspended  by  a  thread. 


28  QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

What  is  an  Artificial  Magnet  f 

If  a  piece  of  iron  or  steel  be  rubbed  or  brought  in 
contact  with  a  loadstone,  or  any  magnet,  it  will  be 
found  to  have  acquired  all  the  properties  of  the  mag- 
net, in  fact  it  has  become  a  magnet  and  if  it  be  hard- 
ened steel  with  proper  care  it  will  retain  the  magnet- 
ism for  an  indefinite  period.  A  piece  of  steel  or  iron 
may  also  be  magnetized  by  winding  insulated  copper 
wire  around  it  and  sending  a  current  of  electricity 
through  the  wire. 


ELECTRO  MAGNET. 

What  is  a  ^Permanent  Magnet  f 

A  magnet  which  holds  its  magnetism  indefinitely. 
A  piece  of  steel  made  glass  hard  and  then  magnetized, 
becomes  a  permanent  magnet. 
What  is  an  Electro  Magnet? 

A  magnet  produced  by  passing  a  current  through  a 
coil  of  wire  around  a  soft  iron  core.  The  core  is  mag- 
netized while  the  current  flows,  but  loses  its  magnet- 
ism when  the  current  stops.  This  form  of  magnet 
may  be  made  much  more  powerful  than  a  permanent 
magnet,  and  is  therefore  used  in  place  of  the  latter 
in  dynamos. 


XII J  MAGNETISM.  29 

\\'/t<tf  /.<  Jf <i</-/t(;(  Satotrati 

\Vhen  a  magnet  has  been  magnetized  to  the  high- 
legree  it  can  attain  it  is  said  to  be  " saturated" 

\\'/i<if  /.<  Jt<. <;<!•« if  M<I<JI. 

*  iron  after  being  magnetised  retains  a  small 
amount  of  magnetism  after  its  temporary  magnetism 
has  red.  This  small  remaining  charge  is  called 

foal  HKKjntfism" 

Does  a  Permanent  J/"////e/  I,  \hrengthi 

You  may  partially  or  wholly  destroy  the  magnet- 
el  magnet  by  rough  usage,  as  hitting  it, 
or  k  \     It  will  also  lose  its  magnetism 

on  1  '-sg. 

W/.fit  i«  f/,<  sin-Hf/tli  of  a  J/"///"'/ 
ft  i-  a  common  error  for  one  to  suppose  that  the 

n'j-th  of  a  magnet  is  its  lifting  power,  but  thi 
not  so.  The  strength  of  a  magnet  is  the  strength 
of  its  poles  ami  the  strength  of  a  magnet  pole  must  be 
measure'!  by  the  magnetic  force  which  it  exert 
example  :  Supposing  there  are  two  magnets  A  and  B, 
whose  strengths  we  will  compare  by  making  them  each 
act  upon  the  North  pole  of  a  third  magnet,  C.  If  the 
th  pole  of  A  repel-  C  with  twice  as  much  force  as 
that  with  which  the  North  pole  of  B  placed  at  the 
same  distance  would  repel  C,  it  would  prove  that  the 
strength  >e  that  of  B. 

Can  «  M<i<ji,<i  l«t>-<  o/,///  o,,<  J'ole? 

the  two  poles   are  inseparable.     If  we  break  a 
magnet  into  any  number  of  pieces  each  piece  will  b<-- 


30  QUESTIONS   AND   ANSWERS   ABOUT  ELECTRICITY. 

come   an  independent  magnet   with  a   north  and  a 
south  seeking  pole. 
What  is  a  Magnetic  Field? 

It  is  the  space  around  a  magnet  or  it  may  be  the 
space  around  a  conductor  carrying  a  current  of  elec- 
tricity which  is  pervaded  by  magnetic  forces.  Every 
magnet  is  supposed  to  have  what  are  termed  lines  of 
force  running  around  it  and  through  it,  or  to  possess 
what  is  termed  a  magnetic  field. 
Describe  the  Magnetic  Needle  or  Compass. 

The  common  form  of  compass  consists  of  a  small 
steel  needle  fitted  with  a  small  cap  of  brass  or  glass 
by  means  of  which  it  can  be  pivoted  upon  the  sharp 
point  of  a  metal  pin  so  as  to  turn  with  as  little  fric- 
tion as  possible ;  it  is  then  magnetized  when  it  will  set 
itself  into  the  magnetic  meridian,  or  in  other  words 
it  will  seek  a  north  and  south  position.  It  is  placed 
in  a  small  brass  box  and  protected  from  dust,  etc., 
by  a  glass  top.  Under  the  needle  is  a  card  marked 
with  the  "points  of  the  compass." 


VOLTAIC   BATTERIES.  31 


CHAPTER   III. 

VOLTAIC     BATTERIES. 

What  is  the  Voltaic  Pile  f 

The  voltaic  pile  was  named  for  Volta,  its  inventor. 
It  consisted  of  discs  of  copper  and  zinc  laid  together 
in  pairs,  each  pair  separated  from  other  pairs  on  each 
side  by  cloth,  or  paper,  moistened  in  salt  water. 
These  discs  were  placed  in  a  vertical  "  pile,"  a  zinc 
at  one  end  and  copper  at  the  other  acting  as  termi- 
nals of  the  series.  A  wire  taken  from  each  of  these 
end  discs  served  to  lead  the  electric  current  away  and 
provide  for  the  return.  The  chemical  action  of  the 
salt  water  on  the  zinc  developed  an  electric  current, 
which  passed  to  the  copper  in  contact  with  it,  then 
through  the  next  moistened  cloth  to  the  second  zinc, 
and  so  on.  Each  separate  couple  contributes  its  pro- 
portionate part  of  the  total  energy. 

What  is  a  Voltaic  Battery  f 

The  Voltaic  Battery  was  easily  developed  from  the 
Voltaic  Pile.  The  substitution  of  a  vessel  of  salt  water 
for  the  moistened  cloth  was  the  only  difference.  Alter- 
nate copper  and  zincs  had  then  to  be  connected  with 
wires.  Because  the  first  battery  was  arranged  with 
the  glass  cups  in  a  circular  form,  the  invention  was 
called  the  "  crown  of  cups."  One  couple  or  pair  of 


32  QUESTIONS  AND   ANSWERS   ABOUT   ELECTRICITY. 


BUNSEN   CELL. 


GROVE    CELL. 


VOLTAIC   BATTERIES.  33 

plates  does  not  properly  constitute  a  battery.     The 
name  "  cell "  should  be  applied  to  the  unit.     A  "  bat- 
tery" is  a  combination  of  two  or  more  cells. 
Describe  some  Different  Kinds  of  Voltaic  Batteries. 

The  early  forms  of  batteries  were  made  on  the 
"  one  solution "  plan  with  copper  for  one  element, 
zinc  for  the  other,  immersed  in  dilute  sulphuric  acid. 
Wollaston's  is  a  good  example.  The  necessity  for  a 
continuous  working  battery  gave  rise  to  "  double 
solution"  or  two  fluid  cells.  Bunsen  invented  one 
in  which  carbon  and  zinc  formed  the  elements.  The 
carbon  block  was  put  in  a  porous  cup  filled  with 
nitric  acid.  The  zinc  was  outside  this  cup  in  a  glass 
jar  filled  with  dilute  sulphuric  acid.  Grove's  device 
was  the  same  with  the  substitution  of  sheet  platinum 
for  carbon.  Daniel's  cells  had  the  zinc  in  the  porous 
cup  with  the  acid,  while  a  copper  plate  was  outside 
in  a  solution  of  sulphate  of  copper.  The  gravity  cell 
is  a  modification  of  Daniel's  depending  upon  the  dif- 
ferent specific  gravity  of  the  two  solutions  to  keep 
them  separated.  The  Grenet,  or  Bichromate  of  Pot- 
ash, is  a  single  solution  cell.  A  plate  of  zinc  is  held 
between  two  carbon  plates  and  immersed  in  a  solution 
of  dilute  sulphuric  acid  and  bichromate  of  potash. 
Leclanche  and  Samson  cells  use  a  pencil  or  cylinder 
of  zinc,  a  block  of  carbon  surrounded  by  some  form 
of  manganese,  and  immersed  in  a  solution  of  sal- 
ammoniac. 
What  is  an  Accumulator  or  Storage  Cell  f 

The  different  forms  of  cells  just  described  make 
what  are  called  "  primary  "  batteries  because  the  cur- 


34          QUESTIONS   AND   ANSWERS    ABOUT   ELECTRICITY. 


GRAVITY    CELL. 


GRENET   CELL. 


VOLTAIC    BATTERIES. 


35 


rent  originates  directly  in  them.  There  is  another 
class  of  more  modern  development,  called  "  secon- 
dary," storage  batteries,  or  accumulators.  These  can 
generate  no  electricity  of  themselves,  but  have  the 
power  of  holding  and  giving  out  a  current,  after 


LECLANCHE   CELL. 

having  been  "  charged."  This  charging  can  be  done 
with  ordinary  primary  batteries,  but  it  is  cheaper 
to  use  dynamos. 

Storage   batteries  are  far  from   perfect,  returning 
only  about  one-half  of  the  energy  put  into  them.    They 


36        QUESTIONS   AND   ANSWERS    ABOUT    ELECTRICITY. 

are  heavy  and  disagreeable  to  handle  and  require  a 
skilled  electrician  to  maintain  them  in  proper  work- 
ing condition.  The  usual  construction  consists  of  two 
series  of  plates,  alternate  ones  being  connected  to- 
gether. These  plates  are  made  of  lead  "  sponge  "  or 
perforated  sheets  and  the  interstices  filled  with  lith- 
age  or  some  patented  preparation.  A  solution  of 


SAMSON    CELL. 

dilute  sulphuric  acid  is  used,  as  if  they  were  primary 
batteries.  After  having  an  electric  current  sent 
through  them  for  a  certain  length  of  time,  chemical 
changes  take  place  in  the  plates  and  they  will  gener- 
ate a  current  of  electricity  when  placed  in  circuit. 
The  current  flows  in  the  opposite  direction  from 
which  they  were  charged. 


VOLTAIC    BATTERIES. 


37 


How  are  Batteries  Connected  for  Different  Purposes  f 
The  different  arrangements  in  which  cells  or  bat- 
teries may  be  connected  together  depend  upon  the 
conditions  of  the  work  to  be  done:  The  old  expres- 
sions, which  are  very  simple  were  that  the  connec- 
tions were  for  "  intensity  "  or  "  quantity."  Each  cell 


STORAGE    CELL. 

possesses  a  certain  energy,  measurable  in  "volts"  and 
"  amperes,"  or  "  intensity"  and  quantity.  If  the  car- 
bon of  one  cell  is  connected  to  the  zinc  of  the  next 
and  so  on,  the  total  available  "intensity"  will  be  the 
sum  of  the  whole  series.  If  the  carbons  are  all  con- 


38          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

nected  together,  and    the    zincs  together  by  them- 
selves, the  "  quantity  "  will  be  the  sum  of  the  whole, 

What  is   Connecting  in  Multiple  f 

The  latter  arrangement  described  above  is  called 
connecting  in  "  multiple."  That  is,  they  are  a  multi- 
plicity of  paths,  a  part  of  the  current  in  each.  The 
effect  is  the  same  as  if  one  large  cell  were  made  with 
plates  of  an  area  equal  to  the  sum  of  the  individual 
cells.  The  intensity  or  "  voltage "  of  such  an  ar- 
rangement is  small,  being  that  of  one  cell.  Hence 
the  current  can  travel  but  short  distances,  only  a  few 
feet,  and  except  for  use  with  large  wires  this  arrange- 
ment should  not  be  adopted.  The  quantity  or  "  am- 
page"  (number  of  amperes)  being  large,  deflagrating 
effects  can  be  produced.  It  is  seldom  that  batteries 
are  connected  in  this  way  for  practical  purposes. 

What  is   Connecting  in  Series? 

When  the  zinc  of  one  cell  is  connected  to  the  cop- 
per or  carbon  of  the  next,  all  the  current  has  to  go 
through  the  cells  in  "  series"  hence  the  designation, 
coupled  in  "  series."  Such  is  the  usual  arrangement 
in  order  to  get  as  high  "  voltage "  as  possible. 
Telegraph  and  fire  alarm  systems  are  illustrations  of 
this  arrangement,  often  having  several  hundred  cells 
in  one  circuit. 

What  is   Connecting  in  Multiple  Series? 

Combinations  of  these  two  methods  make  two  other 
arrangements  technically  designated  as  "multiple 
series,"  and  "series  multiple."  It  is  not  often  that 
batteries  are  coupled  in  these  ways,  but  -it  is  neces- 


VOLTAIC  BATTERIES.  39 

sary,  sometimes  to  secure  a  certain  strength  of  cur- 
rent. Suppose  for  instance  there  are  at  hand  eight 
cells  each  capable  of  supplying  two  volts  and  three 
amperes  and  it  is  desired  to  send  through  the  wires 
in  circuit  12  amperes  under  a  pressure  of  four  volts. 
By  connecting  the  cells  together  in  series,  groups  of 
two,  the  right  voltage  can  be  secured.  The  groups 
can  then  have  all  their  copper  terminals  connected  to 
one  wire,  the  zincs  to  the  other,  and  twelve  amperes 
of  current  will  result.  The  individual  cells  are  in 
series,  the  groups  in  multiple  and  arrangement  called 
multiple  series.  The  method  of  connecting  the  five 
incandescent  lamps  together  in  electric  cars  is  an 
illustration  of  this  arrangement.  In  the  early  days 
of  incandescent  lamps  it  was  not  uncommon  to  sup- 
ply them  from  the  arc  light  circuits.  The  current, 
after  lighting  several  arc  lamps  would  arrive  at  a 
group,  usually  eight,  of  incandescents.  These  lamps 
were  connected  in  multiple  so  that  only  one-eighth 
of  the  current  went  through  each.  Thus  the  lamps 
were  in  series  with  the  rest  of  the  circuit,  but  in  mul- 
tiple with  each  other,  so  the  arrangement  was  called 
series  multiple. 

What  are   Chemical  Actions  which  Take  place  in  a 
Cell? 

A  current  of  electricity  can  be  produced  in  battery 
only  under  conditions  that  the  two  electrodes  differ 
in  "  electric  state."  That  is,  one  shall  be  acted 
upon,  or  decomposed  and  the  other  unaffected  by  the 
surrounding  liquid.  The  more  rapid  the  disintegra- 
tion of  the  active  metal  the  greater  the  current. 


40          QUESTIONS   AND   ANSWERS   ABOUT  ELECTRICITY. 

When  a  battery  is  in  operation,  the  zinc  is  slowly  dis- 
solved ;  the  solution  becomes  charged  with  some  solu- 
tion (usually  sulphate)  of  zinc.  When  all  the  acid 
has  been  used  up,  the  solution  is  entirely  a  "salt," 
as  long  as  the  free  acid  exists  the  battery  may  be  said 
to  be  energetic,  but  having  a  diminishing  strength. 
When  the  "  salt "  alone  is  present  the  electro-motive 
force  is  reduced  but  is  constant.  It  is  under  these 
conditions  that  the  "  gravity "  cells  derive  reputa- 
tion of  constancy. 

What  is  Local  Action  in  a  Cell? 

If  zinc  was  chemically  pure  and  the  acids  pure 
there  would  be  no  action  on  the  zinc  except  that  for 
which  the  electric  current  could  account.  But  there 
are  traces  of  iron,  copper,  lead  and  other  metals  in 
the  zinc  which  form  independent  circuits  by  them- 
selves. This  action  serves  only  to  diminish  the 
strength  of  the  acid  by  setting  up  a  counter  electro- 
motive force  and  dissolve  the  zinc  uselessly. 

Why  is  the  Zinc  Used  in  a    Cell  Amalgamated,  and 

How  is  it  Done  ? 

A  remedy  for  this  "  local  action,  is  amalgamation  of 
the  zinc.  This  is  an  expression  for  coating  the  sur- 
face with  mercury.  With  this  application,  ordinary 
zinc  acquires  the  properties  of  the  pure  metal  and  is 
unattached  detrimentally  by  the  acid.  A  simple  way 
of  amalgamating  is  as  follows  :  Clean  the  zinc  by 
scouring,  and  washing  in  dilute  sulphuric  acid. 
Rinse  the  zinc  in  clean  water  without  touching  it  with 
the  fingers,  and  immerse  it  in  a  vessel  of  mercury. 


VOLTAIC    BATTERIES.  41 

Another  method  is  to  dissolve  one-half  pound  of 
mercury  in  two  pounds  of  nitric  acid.  After  cleaning 
the  zinc,  as  before,  an  immersion  for  a  few  minutes  in 
this  mixture  will  coat  the  zinc  with  a  uniform  layer  of 
mercury.  A  little  mercury  kept  in  the  bottom  of  bat- 
tery cells  will  serve  to  keep  the  zincs  constantly 
amalgamated. 

What  is  Polarization  and  What  are  JKemedies 
Usually  Employed  to  Prevent  it  ? 
As  acid  attacks  the  zinc  plate  in  a  simple  battery 
cell,  hydrogen  bubbles  are  liberated,  these  bubbles 
travel  through  the  liquid,  and  upon  arriving  at  the 
copper,  or  carbon  plate,  they  cover  in  a  short  time 
its  entire  surface,  and  the  strength  of  the  cell  is 
diminished.  The  reason  is  that  the  hydrogen  forms  a 
kind  of  insulating  shield,  and  the  electricity  cannot 
reach  the  plate  itself.  Further,  a  layer  of  zinc  is  de- 
posited on  the  copper,  still  further  reducing  the  cur- 
rent. Snee  invented  a  method  of  freeing  the  "  nega- 
tive "  plate  by  covering  it  with  finely  divided  plat- 
inum. The  hydrogen  bubbles  escaped  from  these 
points  with  considerable  success.  In  the  Bichromate 
cells  the  bichromate  of  potash  chemically  unites  with 
the  hydrogen.  The  only  successful  method  of  get- 
ting rid  of  this  "polarization"  produced  by  the  hy- 
drogen, is  the  interposition  between  the  two  electro- 
des of  a  porous  diaphram.  Unglazed  pottery  is  best. 
To  increase  the  output  of  the  cell  it  is  usual  in  addi- 
tion to  this  device,  to  put  a  different  solution  inside 
the  porous  cup  from  that  outside.  The  hydrogen  as 
before  is  liberated  at  the  surface  of  the  zinc  and 


42         <tUK0TIOVB   AUD   ANSWERS   ABOl  T   ELI 

reaches  the  porous  cup.  Some  passes  off  upwards 
into  the  air,  the  rest  meets  the  acid  or  solution  in  the 
porous  cup  and  is  taken  into  chemical  union.  The 
L< •(  lanche  and  Samson  are  illustrations  of  remark- 
able successful  one-fluid  batteries.  Their  merit  lies  in 
the  large  area  of  carbon  or  negative  in  com- 

parison with  the  /inc.     It  requires  some  time  for  tl,i- 
large  surface  to  be  coated  with  bubbles,  and  mean- 
while the  cakes   of  manganese,  attached  to  the  <-ar- 
i,  have  chemically  disposed  of  a  good  portion  of 
the  gas.     Still  it  is  not  possible  to  get  a  < -ontinuous 
current   from    them    for    more    than    a    «juart«-r  of   an 
hour.     They   are    particularly  adapted    to   telephone 
and    hell  work,  which    require  only   intermittent    cur- 
vity  batt'  -    almost  altogether  u-ed 

when-  a  con-tanf  and  continuous  current  U  wanted, 
as  for  lire  Indeed  t  he 

if  they  are   not  utili/ed    after   befog  IV 
working    «i<l<r.      Tba]    ai«     d<  anl\ .  cheap   and    ea-ily 
:          u  hite    voliitiofi    ot'    sulphate    of    /inc 
ind   the    /inc  in    the  top    ot'    the    jar,  and    the    blue 
OOpptf  in  the  l»ot t (.in.  -eparated  l»y  ^rax  ity 
i/iakes    a    |.ietl\    and   happv    illuMration  of    sim- 
plicity a  nd  eHicac\  .      |;un-en  and    QTOT6    lotteries  are 
.nit     lo    handle    l'i-om    the    nature    of    the    acids 
j.loved,     and     their     [}§(    i^    attended    with    noxious 
In  inc.   \\hidi   are  dan^eroiiH  to   hreatli. 


DYNAMO^    AM)    MOTORS.  43 


CHAPTER  IV. 

DYNAMOS    AND    MOTORS. 

What  is  a  Dynamo t 

A  dynamo  is  a  mechanical  device  for  generating 
electricity.  In  1867  it  was  suggested  by  Siemens 
and  by  Wheatstone  that  a  coil  of  wire  rotating  be- 
tween the  poles  of  an  electro-magnet  might  from  the 
residual  magnetism  induce  a  small  current  of  electrici- 
ty which  when  transmitted  through  the  coils  of  the 
electro-magnet  might  exalt  its  magnetism  and  so  pre- 
pare it  to  induce  still  stronger  currents.  It  is  certain 
that  electric  currents  derived  from  chemical  action  in 
battery  cells  differ  in  nature  from  those  produced  by 
mechanical  motion  in  dynamos.  In  results  however, 
there  is  no  perceptible  difference.  The  current  from 
batteries  is  uniformly  constant  and  regular  in  its 
flow.  From  dynamos  the  current  can  be,  at  will, 
uniformly  continuous,  pulsating,  and  even  alternating. 
Electric  light  and  power  were  not  practicable  until 
dynamos  were  invented.  The  electric  currents  at- 
tainable from  batteries,  is  extremely  meagre  and 
costly,  and  except  where  very  small  amounts  are 
needed,  as  in  telegraph  and  telephone  apparatus, 
have  proved  their  utter  incapacity.  A  dynamo 
needs  simply  to  be  driven  at  a  certain  rate  of  speed 
by  a  steam  engine,  or  water-wheel,  and  while  revolv- 


44          QUESTIONS   AND   ANSWEES   ABOUT   ELECTRICITY 


DIRECT  CURRENT  DYNAMO. 


DYNAMOS   AND   MOTORS.  45 

ing  currents  of  electricity  are  developed,  which  can 
be  collected  by  suitable  appliances  and  carried  away 
by  wires  to  lamps,  motors,  etc.  The  currents  are  due 
to  magneto-electric  induction.  There  are  two  types 
of  dynamos — the  continuous  curretn  and  the  alternat- 
ing current.  In  the  continuous  current  dynamo  the 
current  generated  always  flows  in  the  same  direction, 
there  being  a  commutator  from  which  the  current  is 


ALTERNATING   CURRENT  DYNAMO. 

collected  by  brushes.  In  the  alternating  current  dy- 
namo the  current  generated  flows  at  rapid  intervals, 
first  in  one  and  then  in  the  opposite  direction.  The 
field  magnets  of  this  machine  must  have  a  continuous 
current  to  excite  them  and  this  current  is  generated 
by  a  small  continuous  current  machine  which  is  called 
the  exciter.  There  is  no  commutator  but  a  col- 


46          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

lector  consisting  of  two  metal  rings  upon  which  the 
brushes  rest.  Dynamos  have  been  made  in  which  no 
iron  at  all  was  in  the  armature,  but  such  machines 
have  a  small  output  of  electrical  energy.  Some  ma- 
chines have  been  made  in  which  the  armature  is  sta- 


ELECTRIC    MOTOR. 

tionary,  the  field  magnets  of  the  dynamo  revolving,  but 
they  are  as  yet  experimental.  Usually  the  armature 
forms  the  inside  or  the  centre  of  the  machine  ;  the 
mechanical  construction  is  then  easiest  and  the  more 
delicate  parts  protected  from  injury. 


DYXAMOS   AND   MOTORS.  47 

What  is  an  Electric  Motor? 

An  electric  motor  is,  commonly  speaking,  the  re- 
verse of  a  dynamo.  A  dynamo,  driven  by  mechani- 
cal power  develops  electricity  ;  a  motor  absorbs  the 
electric  current  from  a  dynamo,  and  transforms  elec- 
trical power  into  mechanical  power;  restores,  as  it 
were,  the  original  power  that  actuated  the  dynamo. 
It  is  erroneous  to  suppose  that  a  motor  will  give  back 
more  or  even  as  much  power  as  was  expended  in 
driving  the  dynamo.  Friction  of  bearings,  resistance 
of  wires,  and  energy  expended  magnetising  the  iron 
cause  various  losses  which,  from  the  principles  of  con- 
servation of  energy,  are  insurmountable. 
What  is  an  armature  ? 

In  the  usual  forms  of  dynamos  and  motors  the  re- 
volving part  is  called  an  "  armature."  This  name  is 
borrowed  from  the  phraseology  adopted  in  days  when 
horse  shoe  permanent  magnets  were  invented.  The 
piece  of  soft  iron  that  is  kept  across  the  poles  of  a 
magnet  sometimes  called  the  " keeper"  is  technically 
defined  as  the  armature.  Its  use  is  to  provide  a  path 
for  lines  of  magnetic  force  as  they  travel  from  one 
pole  piece  to  the  other.  The  armature  of  'a  dynamo 
or  motor  consists  of  an  iron  core  or  cylinder  mounted 
upon  a  suitable  shaft.  Copper  wires  are  wound  over 
the  surface  of  this  cylinder.  As  the  magnetism 
travels  through  the  iron,  currents  of  electricity  are 
generated  in  the  wire,  if  the  case  at  hand  is  a  dynamo ; 
or  if  it  is  a  motor  the  wires  serve  to  convey  the  cur- 
rent from  its  source.  There  are  several  ways  of  ar- 
ranging the  coils  of  wire  upon  an  armature,  viz : 


48          QUESTIONS   .AND  AXSWEHS   ABOUT   ELECTRICITY. 


w 

o; 

H 

3 

5 
8 


DYNAMOS   AND  MOTORS.  49 

Drum  Armatures  in  which  the  coils  are  wound  longi- 
tudinally upon  the  surface  of  a  cylinder  or  drum  ; 
Ring  Armatures  in  which  the  coils  of  wire  are  wound 
around  a  ring  instead  of  going  completely  around 
the  outside  of  the  armature  core,  each  turn  of  wire 
going  through  the  opening  in  the  middle  and  thence 
back  to  the  outer  surface ;  Pole  Armatures  in  which 
the  coils  are  arranged  radially  with  their  poles  point- 
ing outwards,  and  Disc  Armatures  having  coils 
arranged  in  or  on  a  disc. 

What  are  Field  Magnets  upon  Dynamos  or  Motors  ? 
Field  magnets  of  dynamos  are  the  "inductors." 
"That  is,  they  "  induce  "  or  persuade  the  current  to  be 
generated.  They  are  simply  large  and  somewhat 
peculiarly  shaped  electro-magnets.  The  first  dyna- 
mos were  made  with  permanent  steel  magnets  for  the 
inductors,  and  from  that  circumstance  derive  their 
name,  magneto-machines.  Such  machines  are  still  in 
use  as  in  some  medical  apparatus  and  in  telephone 
call  bells.  For  a  given  size  electro-magnets  are  many 
times  stronger  than  permanent  ones;  besides  iron  is 
cheaper  than  steel  and  more  easily  put  in  any  desired 
shape.  Permanent  magnets,  so  called,  gradually  lose 
their  strength,  and  the  difficulty  of  "  charging "  them 
is  serious.  Electro-magnets  do  not  lose  their  strength 
by  use,  but  in  most  dynamos  the  regulating  qualities 
depend  upon  a  variation  of  the  magnetism  in  the 
inductors.  The  ends  of  the  field  magnets  are  circu- 
lar in  form,  and  come  very  close  to  the  armature,  so 
that  the  air  gap,  through  which  the  magnetism  must 


50          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 


SECTION   OF    RIN(.    A  KM  All   KK 


RING    ARMATURE    SHOWING    METHOD   OF    WINDING. 


DYNAMOS   AND   MOTORS.  51 

go  to  pass  through  the  iron  core  in  the  least  possible 
distance.  Cast-iron  is  usually  employed  for  field 
magnets,  but  it  is  common  to  see  wrought  iron  form- 
ing  part  of  the  magnetic  circuit.  The  "pole  pieces  " 
which  embrace  the  armature  are  best  made  of  cast- 
iron,  as  they  will  hold  some  "residual"  magnetism, 
and  serve  to  "start"  the  dynamo.  Wrought  iron 
"  cores "  over  which  the  wire  is  wound  makes  very 
strong  magnets.  A  cast  or  wrought  iron  block  to 
connect  the  ends  of  the  magnets  furtherest  from  the 
armature  is  called  the  magnetic  "  yoke."  Sometimes 
the  "  fields"  are  used  in  a  double  capacity  i.  e.,  to  fur- 
nish magnetism,  and  to  give  mechanical  strength  to 
the  machine ;  this  principle  is  simply  an  economy  of 
material.  Dynamos  or  motors  are  simplest  made 
with  a  single  magnet,  having  the  two  poles  N  and  S, 
Multipolar  machines  are  fast  coming  into  the  market 
having  several  poles,  alternately  N  and  S  arranged 
symmetrically  around  the  armature.  Alternate  cur- 
rent machinery  needs  many  poles  in  the  magnetic  in- 
ductors. 
What  is  a  Commutator  and  WJiat  is  its  Use  ? 

As  a  coil  of  wire  that  is  wound  upon  an  armature, 
leaves  one  pole  piece,  a  current  is  induced  in  the  wire 
in  a  certain  direction,  as  the  same  coil  continues  its 
revolution  and  approaches  the  next  pole,  a  current  is 
induced  in  the  opposite  direction.  A  "  commutator  " 
is  a  device  to  set  all  the  alternate  currents  flowing  in 
one  direction ;  the  commutator  consists  of  separate 
bars  or  segments  of  a  circle,  in  number  equal  to  the 
number  of  armature  coils.  Each  coil  is  successively 


52          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

connected  to  a  different  bar,  and  as  these  bars  are  all 
carefully  insulated  from  each  other  the  current  gener- 
ated in  one  coil  cannot  interfere  with  that  generated 
in  any  other.  The  segments  or  bars  are  usually  of 
copper,  as  that  metal  is  one  of  the  best  conductors  of 
electricity.  The  segments  are  held  in  place  by  means 
of  rings  at  the  ends  leaving  the  outer  surface  avail- 
able to  run. 

What  is  the   Use   of  ^Brushes  upon  a  Dynamo   or 

Motor  ? 

The  "brushes."  The  name  of  these  necessary 
adjuncts  to  dynamo  and  motor  is  due  to*  the  brush- 
like  form  in  which  the  wires  are  grouped  together. 
By  making  the  "  brushes  "  of  wires  or  thin  leaves  of 
copper,  a  large  number  of  points  are  in  contact  with 
the  commutator  bars.  The  use  of  brushes  is  to  col- 
lect the  current  that  is  induced  in  the  armature  wind- 
ing, and  after  the  current  has  done  its  work,  return  it 
to  the  armature.  In  other  words  the  brushes  provide 
a  path  into  the  revolving  armature  and  out  again.  In 
dynamos  for  supplying  arc  lamps,  the  current  is  small 
and  the  brushes  consist  of  one  layer  of  sheet  copper, 
about  .3^  in  thickness;  dynamos  for  running  incan- 
descent lamps  supply  a  large  current,  and  the  brushes 
are  made  of  many  layers  of  fine  copper  wires  or 
copper  foil. 

What  is  the  Difference  Between  a  Ring  and  a  Drum 

Armature  ? 

The  distinction  between  "ring"  and  "drum"  arma- 
tures is  based,  not  upon  the  form  of  the  core  so  much 
as  upon  the  ma  nner  of  winding  the  conducting  wires 


DYNAMOS   AND   MOTORS.  53 

In  the  past,  ordinary  drum  armatures  had  solid  cores, 
that  is,  the  sheet  iron  discs  of  which  the  core  consists 
rested  directly  upon  the  shaft.  All  small  machines 
with  drum  armatures  are  constructed  this  way.  The 
wires  are  wound  back  and  forth  lengthwise  of  the  core, 
passing  over  the  ends  close  to  the  shaft  to  get  from  one 
side  to  the  other.  On  the  surface,  the  wires  resemble 
the  arrangement  of  the  tightening  strings  on  a  drum, 
hence  the  name.  In  recent  large  dynamos,  the  mass 
of  iron  would  be  entirely  too  large  if  extended  to  reach 
the  shaft,  and  the  cores  are  hollow,  or  rather  made  in 
the  form  of  rings,  with  the  wire  still  wound  on  the 
outside  surface  only.  Ring  armatures  derived  their 
name  from  the  use  of  ring  core  such  as  has  just  been 
described ;  but  the  winding  is  threaded  in  and  out, 
first  on  the  outer  surface,  then  down  the  side,  through 
the  ring,  up  the  side  again,  and  alongside  the  first 
turn.  Thus  the  entire  outer  and  inner  surfaces  of  the 
ring  are  completely  covered.  Ring  armatures  first 
received  practical  application  in  dynamos.  Gramme 
borrowed  the  idea  from  Pacinotti,  and  this  class  of 
armatures  is  commonly  known  as  the  "  Gramme  ring." 
Siemens  developed  the  drum  form  and  his  name  is 
often  used  in  connection  with  it.  The  recent  prac- 
tice of  putting  a  drum  winding  or  a  ring  core  is 
rather  confusing  to  dynamo  nomenclature. 
What  is  a  Collector  Ming  upon  a  Dynamo  ? 

Currents  of  electricity  are  developed  in  an  arma- 
ture in  alternating  impulses.  Hence  an  alternating 
current  is  in  the  more  natural  form.  By  use  of  a 
commutator,  all  these  currents  may  be  set  flowing  in 


54          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

the  same  direction.  Such  a  current  may  be  said  to 
be  "  rectified."  When  it  is  desired  to  use  an  alter- 
nating current,  it  must  be  taken  from  the  armature 
by  means  of  collector  rings,  with  brushes  resting  upon 
them  as  in  the  case  of  a  commutator.  The  wind- 
ing on  the  armature  is  one  continuous  path,  the  dif- 
ferent coils  being  connected  together  except  at  the 
final  ends.  Each  of  these  ends  is  brought  to  a  sepa- 
rate copper  ring  mounted  upon  the  shaft.  These  rings 
are  insulated  from  each  other,  a  brush  upon  each  ring 
serving  to  receive  the  current.  The  current  passes  ofT 
one  brush  and  returns  by  the  other.  An  instant  later 
No.  2  brush  receives  the  current,  returning  by  No.  1, 
and  so  on  alternately,  in  some  machines  making  15,000 
reversals  per  minute. 
What  is  the  Difference  Between  a  Series,  Shunt  or 

a  Compound  Wound  Dynmo  or  Motor  ? 

In  order  to  generate  the  magnetism  in  electro-field 
magnets  a  current  of  electricity  must  be  employed. 
In  the  first,  dynamos,  batteries  or  small  magneto- 
machines  were  used.  Present  alternating  current 
dynamos  usually  receive  current  for  their  fields  from 
small  direct  current  dynamos  called  "  exciters,"  The 
main  dynamo  is  then  said  to  be  "separately  excited." 
It  is  usual  to  have  each  direct  current  dynamo  excited 
by  current  from  its  own  armature.  Sometimes 
it  is  necessary  to  send  the  entire  current  developed  in 
the  armature  around  the  field  magnets ;  in  this  case 
the  wire  should  be  somewhat  larger  than  that  on  the 
armature.  Such  a  field  winding  is  called  "series." 
Instead  of  sending  all  the  current  around  the  field,  a 


DYNAMOS   AND   MOTORS. 


55 


small  part,  one  or  two  per  cent  of  the  entire  current, 
can  be  set  aside  for  this  sole  purpose.  Then  the  wire 
is  fine  and  the  small  amount  of  current  is  compensated 
for  by  making  many  turns  around  the  iron  of  the 
field.  The  current  thus  consumed  does  not  go  away 


SERIES  WOUND  DYNAMO. 

from  the  dynamo  at  all,  but  has  a  circuit  of  its  own 
apart  from  the  lamps  or  motors  supplied  by  the  main 
part  of  the  current.  Such  a  field  winding  is  called 
"  shunt.''  Combinations  of  shunt  and  series  make  a 
"compound"  winding. 


56          QUESTIONS   AND   ANSWERS   ABOUT  ELECTRICITY. 


Q  O  Q  O 


O  O  O  O  O  O  O  OQcT 


I 


SHUNT  WOUND  DYNAMO. 


oooooooooo  rooooo 


COMPOUND   WOUND   DYNAMO. 


DYNAMOS   AND   MOTORS.  57 

For  what  Purpose  are  each  of  these  Different  Wind- 
ings Used? 

Dynamos  for  arc  lighting  are  "  series "  wound. 
As  the  number  of  arc  lamps  in  a  given  circuit  varies, 
the  potential  (in  volts)  must  vary  accordingly.  The 
current  (in  amperes)  must  be  kept  constant  in  order 
that  the  lamps  may  burn  with  a  uniform  brilliancy. 
This  "  regulation  "  can  be  effected  by  making  the  field 
magnetism  depend  upon  the  number  of  lamps  burning. 
The  "  series  "  winding  accomplishes  this  result.  For 
incandescent  lamps  and  plating,  the  potental  (in 
volts)  should  remain  constant,  while  the  current  (in 
amperes)  should  increase  as  the  load  increases.  The 
field  magnetism  should  remain  constant  or  nearly  so. 
The  "shunt "  winding  furnishes  a  constant  magnet- 
ism, practically  independent  of  the  real  load  of  the 
dynamo.  Shunt  winding  as  just  explained  will  make 
a  dynamo  supply  its  current  at  a  nearly  constant  poten- 
tial. If  a  few  c<  series  "  turns  are  added  the  regula- 
tion becomes  exact  and  automatic.  For  losses  due 
to  resistance  of  wires  and  slipping  of  belts  as  dyna- 
mos increase  their  load  or  work  there  are  a  few  extra 
series  turns  put  on.  This  enables  the  dynamo  to  raise 
its  potential  slightly  to  cope  with  all  demands.  For 
instance,  ordinary  incandescent  dynamos  run  at  110 
volts  when  few  lamps  are  burning.  As  the  full  num- 
ber is  added,  the  potential  should  raise  to  about  125 
volts  at  the  dynamo,  in  order  that  the  lamps  some 
distance  away  may  receive  110  volts.  Such  winding 
is  called  "  compound  "  and  is  receiving  extended  ap- 
plication on  incandescent  and  railway  dynamos. 


58          QUESTIONS  AND   ANSWERS   ABOUT   ELECTRICITY. 


CHAPTER  V. 

ELECTRIC      LAMP  S  . 

What  is  an  Electric  Arc  f 

In  1810  Sir  Humphrey  Davy  discovered  the  phe- 
nomena of  the  electric  arc.  His  description  is  as  fol- 
lows :  "  When  pieces  of  charcoal  about  an  inch  long 
and  one-sixth  of  an  inch  in  diameter  were  brought 
near  each  other  (within  the  thirtieth  or  fortieth  part 
of  an  inch)  a  bright  spark  was  produced,  and  more 
than  half  the  volume  of  the  charcoal  became  ignited 
to  whiteness;  and  by  withdrawing  the  points  from 
each  other,  a  constant  discharge  took  place  through 
the  heated  air  in  a  space  equal  at  least  to  four 
inches,  producing  a  most  brilliant  ascendant  arc  of 
light,  broad  and  conical  in  form  in  the  middle."  Of 
course  the  light  did  not  last  long,  as  the  charcoal, 
being  soft,  burned  rapidly  away.  The  necessary  cur- 
rent was  supplied  by  a  battery  of  2000  cells,  with  zinc 
and  copper  plates,  the  exciting  fluid  being  dilute  sul- 
phuric and  nitric  acids.  Davy  touched  the  charcoal 
points  together  horizontally  after  attaching  the  wires 
to  the  battery,  and  then  separated  them.  The  stream 
of  hot  flames  which  followed  or  joined  the  points 
were  deflected  by  air  currents  and  took  the  form  of 
an  arch  or  curve,  which  gave  the  name  to  the  phe- 


ELECTRIC    LAMPS. 


59 


THE   ELECTRIC   ARC. 


60          QUESTIONS   AND   ANSWERS    ABOUT   ELECTRICITY. 

nomena.  *"  The  arc  proper  is  composed  of  a  stream 
of  vapor  arising  from  the  actual  boiling  or  vaporiza- 
tion of  the  solid  or  fused  ends  of  the  separated  con- 
ductors. In  so  far  as  the  surrounding  air  mixes  or 
combines  with  this  vapor  stream,  it  is  modified  by  the 
presence  of  oxygen  and  nitrogen,  but  the  air  or  any 
other  gas  is  not  essential  to  be  present,  and  is  merely 
incidental  to  the  formation  of  the  true  arc  stream  in 
air." 
Describe  the  Arc  Lamp. 

The  Arc  Lamp  now  in  general  use  consists  sim- 
ply of  two  hard  carbon  rods,  one  above  the  other, 
and  a  mechanical  contrivance  to  feed  them.  It  is 
necessary  that  the  mechanism  should  start  the  arc  by 
causing  the  pencils  to  touch  and  then  separate  them 
to  the  requisite  distance  for  the  production  of  a 
steady  arc ;  it  should  also  cause  the  carbon  to  be  fed 
into  the  arc  as  fast  as  they  consume,  and  to  approach 
or  recede  automatically,  in  case  the  arc  becomes  too 
long  or  too  short ;  it  should  further  bring  the  carbons 
together  for  an  instant,  to  start  the  arc  again  if  it 
should  go  out.  There  are  a  great  many  forms  of  arc 
lamps,  but  the  one  in  general  use  by  a  number  of  the 
large  companies  is  the  clutch  lamp.  This  is  a  simple 
device,  consisting  of  a  clutch  to  pick  up  the  upper 
carbon  holder,  the  lower  carbon  remaining  fixed.  The 
clutch  is  worked  by  electro-magnets,  through  which 
the  current  passes.  If  the  lamp  goes  out  the  magnets 
release  the  clutch,  and  the  upper  carbon  falls  by  its 
own  weight  and  touches  the  lower  carbon.  Instantly 

*Paper  read  by  Prof.  Elihu  Thomson  at  Providence,  R.  I. 


ELECTRIC    LAMPS. 


61 


ABC  LAMP. 


62          QUESTIONS   AND   ANSWERS   ABOUT  ELECTRICITY. 

the  current  starts  round  the  electro-magnets,  caus- 
ing it  to  act  on  the  clutch,  which  grips  the  carbon- 
holder  and  raises  it  to  the  requisite  distance.  When 
the  arc  grows  too  long  the  lessening  attraction  on  the 
clutch  permits  the  carbon-holder  to  advance.  In  a 
double-carbon  lamp  when  one  carbon  has  burned  out 


INCANDESCENT  LAMP. 


the  current  is  shifted  automatically  to  the  other,  thus 
making  the  life  of  the  lamp  twice  as  long. 
Describe  the  Incandescent  Lamp. 

The  lamp  consists  of  a  glass  globe  or  bulb,  from 
which  the  air  has  been  exhausted,  containing  a  car- 
bonized fiber  of  bamboo.  The  carbonized  fiber  is  at- 


ELECTRIC     LAMPS. 


63 


tached  to  two  platinum  wires  fused  in  the  glass,  the 
free  ends  of  the  wires  being  connected  to  the  copper 
sockets  of  the  lamp,  which  are  insulated  from  each 
other  by  Plaster  of  Paris.  The  wires  are  then  con- 
nected to  the  external  circuit.  Figure  1  is  a  sectional 
view  of  a  familiar  type  of  the  16  candle-power  lamp, 


c  being  the  carbon  loop  ;  b  the  glass  bulb ;  f  the  col- 
lar; and  w  w  the  platinum  leading  wires.  In  order 
to  produce  light  the  carbon  loop  is  brought  to  a  white 
heat  or  incandescence,  by  the  heat  energy  of  an  elec- 
tric current.  No  known  substance  will  endure  this 
temperature  of  white  heat  without,  in  time,  disinte- 
grating, but  carbon  seems  to  stand  this  white  heat 


64          QUESTIONS    AND    ANSWEItS    ABOUT    ELECTRICITY. 

longer  than  any  other  substance  especially  when  it  is 
enclosed  in  a  high  vacuum.  But  all  incandescent 
lamps  "  fail  "  after  running  a  certain  number  of  hours, 
because  their  filaments  disintegrate — they  do  not 
burn  at  the  temperature  of  incandescence.  The  aver- 
age life  of  an  incandescent  lamp  is  from  six  hundred 
to  one  thousand  hours,  after  that  the  filament  be- 
comes worthless  and  must  be  renewed.  The  carbon 
loop  is  usually  made  from  some  vegetable  fiber,  such 
as  is  obtained  from  bamboo  or  some  plant  possessing 
a  similar  structure.  Silk  is  used  by  some  manufactur- 
ers but  as  yet  has  not  proved  to  give  as  good  results 
as  bamboo.  As  the  object  is  to  obtain  a  compact  car- 
bon, a  fiber  rich  in  this  element  is  the  best.  It  must 
have  a  straight  grain  and  possess  great  tensile 
strength  in  order  to  endure  the  process  of  reducing 
its  size  to  the  required  proportions.  This  process 
consists  of  shaving  or  drawing  them  through  a  series 
of  dies,  each  die  removing  a  small  portion  of  the  sur- 
face until  the  proper  diameter  is  obtained.  This  di- 
ameter is  varied  according  to  the  candle-power  and 
voltage  of  the  lamp.  Having  brought  the  fibres  to 
the  proper  size,  they  are  then  baked  or  carbonized  by 
a  method  similar  to  gas  making,  except  that  the  resi- 
due is  the  article  particularly  desired,  and  not  the 
gases  that  are  driven  off.  The  fibre  is  now  bent 
around  a  graphite  block  so  as  to  form  a  loop,  a  num- 
ber of  fibres  being  put  on  each  block,  which  are  then 
packed  in  a  pot  containing  carbon  dust  and  subject  to 
the  intense  heat  of  a  furnace  for  a  number  of  hours. 
During  this  process  air  must  be  excluded  from  the 


ELECTRIC    LAMPS.  65 

pot  containing  the  fibres,  otherwise  a  certain  portion 
of  their  structure  will  be  burned.  Having  properly 
carbonized  the  fiber  it  is  attached  to  the  wires  (w  w) 
which  have  previously  been  fused  into  the  glass  (s)  ; 
this  is  now  enclosed  in  the  glass  bulb  and  the  lamp 
is  now  ready  to  be  connected  to  the  vacuum  pump 
by  means  of  which  the  bulb  is  exhausted  of  air. 


66          QUESTIONS    AND    ANSWERS    ABOUT   ELECTRICITY. 


CHAPTER  VI. 

MISCELLANEOUS     ELECTRICAL     APPARATUS. 

Describe  an  Electric  Bell. 

The  electric  bell  in  common  use  consists  of  an 
electro-magnet  having  an  armature  to  which  is  at- 
tached a  small  hammer,  so  arranged  that  by  sending 
a  current  through  the  circuit  the  armature  is  made 
to  vibrate  and  the  hammer  beats  against  a  gong. 
The  bell  is  usually  operated  by  a  battery,  and  the 
circuit  may  be  closed  by  simply  pressing  a  small  push 
button,  thus  causing  the  current  to  flow  along  the 
line  and  around  the  coils  of  the  electro-magnet,  which 
draws  the  armature  toward  it.  A  contact  breaking 
consists  of  a  spring,  tipped  with  platinum,  which  rests 
against  a  platinum  tipped  screw  is  attached  to  the 
armature.  This  makes  and  breaks  the  circuit  which 
causes  the  hammer  to  beat  against  the  gong  and  then 
fall  back  against  the  screw  which  immediately  closes 
the  circuit  again,  causing  the  armature  again  to  be 
attracted  toward  the  magnet,  and  so  on.  In  fact  an 
electric  bell  is  a  minature  electric  motor. 
Describe  the  Electric  Telegraph. 

The  principles  of  electro-magnetism  early  suggested 
the  possibilities  of  transmitting  messages  by  means  of 
wires.  That  the  electro-magnet  could  be  at  a  long 
distance  from  the  battery,  or  that  an  operator  by 


MISCELLANEOUS   ELECTRICAL   APPARATUS.  67 

merely  touching  two  wires  together,  could  cause  a 
movement  in  an  electro-magnet  a  long  distance  away 
was  a  fact  not  long  left  undeveloped.  Early  it  was 
proposed  to  use  26  or  more  separate  wires  and  send 
impulses  of  electricity  over  these  in  the  order  desig- 
nated by  the  letters  of  the  words  to  be  spelled.  The 
expense  of  this  large  number  of  wires  happily  com- 
pelled the  invention  of  some  cheaper  device.  Prof. 
Morse  found  that  two  wires  would  be  sufficient,  the 
different  letters  to  be  distinguished  from  each  other 
by  allowing  short,  long  or  interrupted  impulses 
of  current  to  be  sent  over  the  lines.  When  the  wires 
were  touched  together  by  means  of  a  "key"  at  the 
sending  station  the  current  would  go  away  to  the 
end  of  the  wires  and  attract  the  armature  of  an 
electro-magnet  at  the  receiving  station,  on  this  arma- 
ture is  a  needle  point  that  punctures  a  paper  strip 
kept  travelling  beneath  by  means  of  cloth  work.  If 
the  current  passes  for  an  instant  only,  a  prick  is  made 
through  the  paper,  called  a  Ci  dot,"  if  for  a  longer 
time  a  slit  or  "  dash."  By  an  accepted  code  made 
by  combinations  of  dots  and  dashes,  the  whole  alpha- 
bet and  symbols  are  arranged.  Since  this  receiving  in- 
strument records  the  messages,  it  is  called  a  "register." 
Attempts  at  economy  led  to  experiments  with  rail- 
road rails  used  as  one  wire.  The  device  was  success- 
ful, and  it  was  even  found  that  the  earth  was  a  suffi- 
ciently good  conductor.  To  an  experienced  operator 
the  clicks  of  the  electro-magnet  are  as  intelligible  as 
the  record  on  a  strip  of  paper ;  it  is  usual  now  to  dis- 


QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 


MISCELLANEOUS   ELECTRICAL   APPARATUS.  69 

pense   with  the  recording  device  and   depend  upon 
sound  alone;  the  receiver  is  then  called  a  "sounder." 


1 


b 

c 

d 

e 

t 

g 

h 

j 

k 

1 

m 

n 

0 

pqrstuv  w 

x  y          z  &  .  ? 

i  2  3  A 


10 


MORSE  TELEGRAPH  ALPHABET. 

What  is  a  Relay? 

For  long  distances  the  number  of  batteries  required 
to  operate  the  sounder  is  large.  Even  thus  equipped 
with  batteries  the  current  in  very  small  and  can  be 
used  to  operate  a  "relay"  only.  The  relay  is  pro- 
vided with  a  delicately  balanced  armature,  the  motions 
of  which,  under  the  influence  of  the  current,  open 
and  close  a  battery  circuit,  which  is  located  at  the  re- 
ceiving station.  The  sounder  operated  by  these  "local" 
cells  strikes  the  sounder  loud  enough  to  be  heard. 
The  progress  of  telegraphy  has  even  surpassed  the 
use  of  a  special  wire  for  each  operator.  Duplex  and 
quadruplex  systems  allow  two  or  /bwr  messages  to  be 
sent  over  the  same  wire  at  the  same  time  by  different 
operators  without  interference.  A  diagram  of  a 
telegraph  circuit  is  given  in  Fig.  2. 


70          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 


f! 


rr 


a 


B 

- 


2 — ©@@©© — D 


MISCELLANEOUS   ELECTRICAL   APPARATUS.  71 

Describe  the  Telephone. 

It  is  strange  to  think  that  the  telephone  operates  on 
the  same  principle  as  the  telegraph.  The  only  differ- 
ence consists  in  the  delicacy  of  the  telephonic  appara- 
tus, and  the  minuteness  of  the  amount  of  the  electric 
current  employed.  A  telegraphic  message  is  sent  by 
the  movements  of  a  key  in  the  operator's  hand ;  a 
telephonic  message  by  the  vibrations  of  a  diaphram 
when  affected  by  the  undulations  of  the  voice. 
A  simple  telephone,  as  constructed  by  Bell,  con- 
sisted of  a  bar  magnet,  on  one  end  of  which  was  a 
small  coil  of  fine  insulated  wire.  In  front  of  this  end 
and  very  close  but  not  touching  was  a  thin  iron  dia- 
phram, such  as  tintypes  are  made  on.  The  ends  of  the 
wires  were  led  away  to  an  exactly  similar  instrument. 
When  the  vocal  disturbances  in  front  of  one  telephone 
vibrated  the  diaphram,  minute  currents  of  electricity 
were  generated,  which  travelled  over  the  wires  and  so 
affected  the  magnet  in  the  receiving  telephone  that 
similar  vibrations  of  that  diaphram  were  produced 
and  restored,  as  it  were,  the  original  words.  The 
present  telephonic  installation  is  somewhat  more 
complicated,  in  fact,  all  that  the  original  Bell  tele- 
phone is  used  for  is  as  a  receiver.  A  magneto- 
machine  is  used  to  ring  the  bells,  a  Blake  and  Edison 
transmitter  receives  the  message  and  a  battery  to 
supply  a  current. 

What  is  a  Microphone? 

The  real  transmitting  apparatus  of  a  telephonic 
system  is  the  "  microphone."  This  device  takes  the 
meagre  motions  produced  by  the  voice  and  magnifies 


72          QUESTIONS   AND    ANSWERS   ABOUT   ELECTRICITY. 

their  effect.  A  battery  supplies  a  current  which  flows 
as  long  as  the  telephone  receiver  is  off  its  hook.  The 
current  passes  through  two  carbon  buttons  in  suc- 
cession, that  lie  touching  each  other  loosely,  then 
through  the  primary  of  a  small  transformer,  or  induc- 
tion coil  and  back  to  the  battery.  The  primary  of  the 


FIGURE  3. 

coil  is  attached  to  the  line  wires,  or  to  one  wire,  the 
other  to  the  ground.  When,  by  the  voice,  the  dia- 
phram,  against  which  the  carbon  buttons  rest,  vibrates, 
these  buttons  are  compressed  slightly,  or  released, 
which  causes  a  better  or  poorer  path  for  the  battery 
current.  The  transformer  magnifies  the  intensity  of 
these  variations  and  goes  out  over  the  line  into  the 
receiver  at  the  other  end.  An  illustration  of  a  micro- 
phone is  given  in  Fig.  3. 


MISCELLANEOUS   ELECTRICAL   APPARATUS.  73 

What  is  an  Induction  Coil? 

Any  coil,  consisting  of  two  pieces  of  wire  entirely 
distinct  and  independent  of  one  another,  wound  upon 
a  core  generally,  but  not  always  of  iron.  The  passage 
of  an  alternating,  vibratory,  or  intermittent  current 
through  one  of  these  wires,  (termed  the  primary  coil) 
induces  a  current  to  flow  in  the  other  wire,  provided  its 
two  ends  are  joined  through  a  conducting  substance. 
Ruhmkorff  coils,  converters,  transformers  and  weld- 


INDUCTION  COIL. 


ing  machines  are  all  induction  coils.  An  induction 
coil  may  be  made  by  winding  a  short  length  of  rather 
large  wire,  say  No.  16,  and  a  great  length  of  very  fine 
wire  around  a  bundle  of  soft  iron  wire,  carefully 
insulating  the  coils  from  one  another,  and  both  from 
the  iron.  By  passing  the  current  from  a  battery  and 
"  interrupter  "  through  the  heavy  coil,  a  comparatively 
strong  potential  may  be  obtained  from  the  fine  wind- 
ing, sufficient  to  give  a  shock  or  a  bright  spark  when 
the  ends  are  brought  together. 


74          QUESTIONS   AND   ANSWERS   ABOUT  ELECTRICITY. 

What  is  a  Ruhmkorff  Coil  ? 

A  kind  of  induction  coil  invented  by  Ruhmkorff. 
Originally  the  Ruhmkorff  coils  were  made  in  com- 
paratively small  sizes,  as  it  was  found  impossible  to 
insulate  the  secondary  wire  sufficiently  to  resist  the 
high  potential  of  the  electricity.  The  different  layers 
of  wire  were  laid  the  entire  length  of  the  coil,  but 
separated  from  each  other  by  silk.  Ritchie  wound 
-the  secondary  in  sections,  each  on  a  spool  of  the 
finished  diameter  of  the  cell,  but  very  short,  requiring 


KUIIMKOKFF    COIL. 


eight  or  ten  such  spools  in  a  row  to  give  the  requisite 
total  length.  These  spools  were  made  of  hard  rub- 
ber, and  could  stand  very  high  potentials.  Ruhm- 
korff bought  one  of  these  coils,  took  it  to  pieces,  and 
copied  the  construction  in  the  large  sizes  of  his 
manufacture. 

What  is  a  Transformer? 

A  "transformer"  or  "converter"  is  an  induction  coil, 
designed  as  a  rule  to  reduce  high  to  low  pressure  in 
connection  with  electric  lighting. 


MISCELLANEOUS    ELECTRICAL   APPARATUS. 


75 


TRANSFORMER. 


76  QUESTIONS   AND    ANSWERS   ABOUT   ELECTRICITY. 

What   is  the    Principle    of   the    Electric    Welding 

Machine  f 

An  electric  welding  machine  is  a  kind  of  induction 
coil,  having  a  very  short,  heavy  secondary,  generally 
consisting  of  but  one  turn  and  capable  of  carrying  an 
enormously  heavy  current  reduced  by  the  primary. 
Pieces  of  metal  attached  to  movable  ends  of  the 
secondary  are  melted,  or  welded  at  their  ends  by  the 
heavy  current.  The  welding  machine  Is  used  to  weld 
or  unite  pieces  of  metal,  such  as  pipe,  rod,  wire,  etc. 


ELECTRICAL   MEASUREMENT.  77 


CHAPTER  VII. 

ELECTRICAL    MEASUREMENT. 

Ifow  are   Currents  of  Electricity  Measured? 

By  measuring  the  effect  of  a  current  upon  a  mass 
of  iron  or  upon  a  magnetic  needle,  or  upon  another 
current.  Thus,  imagine  a  small  coil  of  wire  with  its 
axis  vertical.  Let  a  bar  of  soft  iron  be  suspended 
vertically  by  a  spring  with  its  end  just  entering  the 
coil.  The  bar  should  be  of  such  size  as  nearly  to  fill 
the  interior  aperture  of  the  coil.  If  now  a  current  is 
passed  through  the  coil  the  bar  will  be  attracted  and 
will  descend  more  or  less  deeply  into  the  coil  as  the 
current  is  stronger  or  weaker.  Such  are  the  essential 
parts  of  an  apparatus  for  measuring  a  current.  The 
movements  of  the  core  indicate  strength  of  the  cur- 
rent. Again  let  a  coil  of  wire  be  placed  around  a 
compass.  The  wire  must  be  carried  across  the  top, 
down  one  side,  across  the  bottom  and  up  the  other 
side.  Then,  if  the  coil  is  turned  north  and  south,  a 
current  passed  through  it  will  make  the  needle  of  the 
compass  turn  aside,  either  to  the  east  or  west.  The 
amount  of  displacement  will  be  greater  as  the  current 
is  greater.  This  is  another  type  of  current  measur- 
ing apparatus.  The  first  described  apparatus  is  a 
solenoid  instrument,  the  second  described  one  is  a 
galvanometer.  In  all  cases,  either  by  trial  or  calcula- 


78  QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

tion,  the  movements  of  a  galvanometer  needle  or  of 
the  other  moving  piece  in  a  current  measurer  must  be 
made  to  give  the  strength  of  current  causing  them. 
What  are  the  Names  of  the  Different  Electrical 
Units  and  What  do  They  Mean? 
The  ampere  is  the  unit  of  current  strength.  It 
means  a  strength  of  current  that  will  decompose 
.09326  milligrams  of  water  in  a  second  ;  that  will  pre- 
cipitate 1.11815  milligrams  of  silver  per  second  from 
a  solution  of  metal.  The  above  weights  are  given 
differently  by  different  authorities.  The  coulomb  is 
the  unit  of  quantity.  It  is  the  quantity  of  electricity 
passed  by  one  ampere  per  second.  Hence  the  values 
just  given  for  the  ampere  are  really  the  values  of  the 
coulomb.  The  volt  is  the  unit  of  electromotive  force. 
It  is  what  produces  a  current  through  a  wire  or  other 
conductor,  just  as  pressure  forces  water  throngh  a 
pipe.  A  gravity  battery,  such  as  seen  in  telegraph 
offices  gives  an  electromotive  force  of  almost  exactly 
one  volt.  The  ohm  is  the  unit  of  resistance.  Just  as 
a  pipe  by  friction  resists  the  passage  of  water,  so  a 
wire  or  other  conductor  resists  the  passage  of  elec- 
tricity. A  large  wire  like  a  large  pipe  resists  it  less 
than  a  small  one.  If  twice  as  thick  as  a  given  wire, 
it  resists  only  one  quarter  as  much.  A  pure  copper 
wire  .325  inch  thick  and  10,000  feet  long,  or  one  .10 
inch  thick  and  1,000  feet  long  has  almost  exactly  one 
ohm  resistance.  The  farad  is  the  unit  of  capacity.  It 
is  the  capacity  of  a  surface  which  at  one  volt  pres- 
sure will  retain  one  coulomb  of  electricity.  Electricity 
acts  like  air.  Twice  as  much  air  as  it  normally  holds 


ELECTRICAL    MEASUREMENT.  79 

can  be  pumped  into  a  given  bottle  if  the  pressure  is 
doubled,  and  three  times  as  much  if  the  pressure  is 
tripled  and  so  on.  So  a  surface  of  one  farad  capacity 
will  hold  two  farads  at  two  volts  pressure,  and  three 
farads  at  three  volts  pressure.  The  capacity  of  a  sur- 
face is  the  same,  whether  it  is  the  surface  of  a  solid 
metallic  object  or  is  merely  coated  with  the  thinnest 
gold  leaf.  There  is  no  such  thing  as  capacity  of  a 
solid  volume,  it  is  purely  superficial.  These  are  the 
five  principal  electric  units  used  in  practice. 
What  is  the  Law  of  Inverse  Squares  ? 

The  law  of  change  of  intensity  of  radiant  energy 
with  distance.  If  light  is  given  by  a  point  or  very 
small  object,  the  light  given  by  it  is  stronger,  the  less 
the  distance.  The  law  of  inverse  squares  state  that 
the  intensity  of  the  light  at  a  distance  of  2  is  one- 
fourth  as  strong  as  at  a  distance  of  1 ;  at  a  distance  of 
8  it  is  one-ninth  as  strong ;  at  a  distance  of  4  it  is 
one-sixteenth  as  strong  and  so  on,  the  denominator  of 
the  fraction  in  each  case  being  the  square  of  the  dis- 
tance expressed  in  terms  of  any  selected  standard 
distance.  The  same  law  applies  to  heat  derived  by 
radiation  from  a  point  or  small  object,  and  to  other 
such  cases.  It  is  the  law  of  gravitation.  While 
theoretically  it  is  only  true  when  applied  to  points,  it 
is  practically  accurate  when  the  objects  are  very 
small  in  proportion  to  their  distances.  This  is  why 
it  appilies  to  the  sun,  the  planets  and  their  satellites. 

What  is  the  Unit  of  Quantity  of  Electricity  f 
The  coulomb  is  the  practical  unit. 


80  QUESTIONS   AND   ANSWERS   ABOUT  ELECTRICITY. 

What  is  the  Unit  of  Pressure  ? 
The  volt  is  the  practical  unit. 

What  is  the  Unit  of  Energy  ? 

The  unit  of  electric  energy  is  a  coulomb  urged  by  a 
pressure  of  one  volt.  It  is  called  the  volt  coulomb. 
But  as  a  rule  what  is  wanted  is  the  rate  of  energy,  not 
the  mere  quantity  of  it.  The  unit  of  activity  or  rate 
of  energy  is  an  ampere  urged  by  a  pressure  of  one 
volt ;  it  is  called  the  volt-ampere  or  the  watt.  Seven 
hundred  and  forty-six  (about)  watts  are  equal  to  one 
horse  power. 

What  is  Ohms  Law? 

The  law  stating  the  relations  of  current  strength, 
electric  pressure  and  resistance.  It  states  the  simplest 
possible  relation,  one  which  has  by  experiment  been 
found  to  be  the  true  one.  It  states  that  the  current 
strength  is  equal  to  the  electric  pressure  divided  by 
the  resistance,  when  all  are  expressed  in  accordant 
units,  such  as  the  ohm,  volt  and  ampere.  Thus,  in  a 
circuit  of  one  ohm  resistance,  with  one  volt  electro- 
motive force  or  electric  pressure,  a  current  of  one 
ampere  will  be  produced.  Again,  if  in  the  same 
circuit  there  were  ten  volts  pressure,  there  would  be 
ten  amperes  of  current ;  if  the  resistance  of  the  circuit 
were  twice  or  three  times  as  great,  the  current  would 
be  one-half  or  one-third  as  strong. 

What  is  a  Resistance  in  a  Circuit  ? 

A  wire  or  other  conductor  acts  for  electricity  as  a 
pipe  does  for  water.  It  resists  its  passage.  If  large 
it  resists  less  than  if  small,  just  as  a  pipe  does.  If 


ELECTRICAL   MEASUREMENT.  81 

twice  as  long  a  wire  resists  twice  as  much  as  another, 
otherwise  the  same.  If  half  as  thick  as  another  wire 
otherwise  the  same,  it  resists  four  time  as  much,  or  in 
inverse  proportion  to  its  cross-sectional  area  .5. 

What  is  Meant  by  Potential  ? 

Electricity  is  evenly  distributed,  except  where 
some  disturbing  cause  crowds  more  into  one  place 
than  into  another.  This  raises  the  potential  of  the 
given  place,  and  the  rise  of  potential,  which  is  really 
the  rise  of  electric  pressure,  tends  to  drive  the  elec- 
tricity back  again.  It  will  force  it  in  a  current 
through  a  conductor  leading  to  a  place  of  less  poten- 
tial. Potential  is  best  thought  of  as  electric  pressure. 

What  is  a  Watt  ? 

The  unit  of  rate  of  electric  work  or  energy.  It  is  a 
current  of  one  ampere  urged  by  a  pressure  of  one 
volt.  It  is  sometimes  called  a  volt-ampere.  (See 
Answer  6). 

What  is  the  Unit  of  Current  Strength  ? 

The  ampere,  already  described.     (See  Answer  2). 
Describe  the    Galvanometer,    its     Construction   and 
Use. 

Wind  a  number  of  turns  of  wire  around  a  wooden 
ring  and  support  it  on  its  edge.  Inside  the  ring  place 
a  compass.  Turn  the  ring  so  that  it  is  in  line  with 
the  magnetic  needle,  and  you  have  a  galvanometer. 
In  the  real  instruments  every  kind  of  modification  is 
introduced.  The  coil  may  have  a  very  large  number 
of  turns,  and  be  made  of  exceedingly  fine  wire,  or  it 
may  be  a  single  turn  of  thick  wire,  or  may  be  repre- 


82  QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

sented  by  a  bent  bar  of  copper.  The  magnetic  needle 
may  be  long  or  short,  and  may  be  poised  on  a  point,  or 
may  be  suspended  by  a  very  fine  filament  or  thread, 
sometimes  so  fine  that  it  is  barely  visible.  In  the 
latter  case  a  little  looking  glass  is  attached  to  the 
needle.  This  reflects  a  spot  of  light,  exactly  as  boys 
do  with  a  hand  mirror  to  annoy  people  in  the  street. 
The  movements  of  this  spot  of  light  are  used  to  show 


GALVANOMETER. 

the  minute  movements  of  the  needle.  If  a  current  is 
passed  through  the  coil  of  a  galvanometer,  the  needle 
will  try  to  turn  so  as  to  point  right  across  the  coil. 
The  extent  of  its  movement  will  depend  upon  the 
strength  of  the  current.  The  galvanometer  is  used 
for  three  general  purposes.  One  is  to  show  if  there 
is  any  electromotive  force  or  potential  difference 
between  two  points.  For  this  use  it  must  be  very 
delicate,  but  in  this  case  it  is  not  a  measuring  instru- 


ELECTRICAL   MEASUREMENT.  83 

ment,  it  is  only  used  as  an  indicator.  The  next  pur- 
pose is  to  measure  currents.  The  needle  will  be 
deflected  more  or  less  in  proportion  to  the  current  and 
the  relation  of  the  deflections  to  the  currents  must  be 
ascertained  for  each  galvanometer.  The  third  pur- 
pose is  to  measure  the  electromotive  force  or  poten- 
tial difference  between  two  points.  A  galvanometer 
with  a  coil  of  a  great  number  of  turns  of  very  fine 
wire  is  used  for  this.  The  deflections  of  the  needle 
are  in  proportion  to  the  current  going  through  the 
coil,  and  this  current  is  proportional  to  the  electro- 
motive force  or  potential  difference  between  the  ends 
of  the  galvanometer  coil.  If,  therefore,  these  ends 
are  connected  to  the  points  whose  potential  difference 
is  to  be  measured,  the  movements  of  the  needle  will 
be  greater  or  less  as  this  potential  difference  is  greater 
or  less.  This  gives  the  basis  for  the  measurement. 
There  are  a  great  number  of  ways  of  conducting 
these  measurements.  A  whole  book  might  be  devoted 
to  the  different  methods  of  measurements,  in  almost 
all  of  which  the  galvanometer  is  used  in  some  way. 
Describe  a  Voltmeter. 

A  voltmeter  is  a  current  measuring  instrument  of 
any  type,  which  is  wound  with  fine  wire  of  very  great 
length.  In  the  first  answer  and  in  the  twelfth  answer 
current  measures  are  described.  Either  one,  with 
proper  graduation,  becomes  a  voltmeter  if  its  coils 
are  of  fine  enough  wire  and  of  high  enough  resist- 
ance. Ayrton's  voltmeter  is  of  the  solenoid  type,  as 
described  in  the  beginning  of  the  first  answer.  The 
attracted  bar  is  suspended  by  a  long  spiral  spring. 


84  QUESTIONS   AND   ANSWERS   ABOUT  ELECTRICITY. 


SPRING  VOLTMETER  AND    SPRING    AMMETER. 


ELECTRICAL   MEASUREMENT.  85 

The  effect  of  this  is  that  as  the  bar  is  drawn  down- 
ward, it  rotates  or  turns  around  its  vertical  axis.  An 
index  like  the  hand  of  a  clock  is  fastened  to  the  top 
of  the  bar,  and  as  the  current  passing  through  the 
coil  attracts  and  draws  the  bar  downwards,  it  twists 
around  and  the  hand  moves  like  the  hand  of  a  clock 
over  a  horizontal  dial.  A  scale  marked  off  by  trial 
for  volts  is  provided  so  that  the  hand  points  to  the 
volts  at  once.  The  coil  is  of  very  fine  wire  with  a 
great  number  of  turns  or  convolutions.  A  current 
passing  through  a  conductor  heats  it ;  a  strong  current 
heats  it  more  than  a  weak  one  does.  Cardew's  volt- 
meter is  based  on  this  principle.  It  includes  a  long 
fine  wire,  through  which  a  current  due  to  the  differ- 
ence of  potential  between  any  two  points  of  a  circuit, 
to  which  it  may  be  connected,  passes.  This  current 
heats  the  wire  more  or  less  and  expands  it.  The 
expansion  moves  an  index  over  a  scale  and  the  move- 
ments of  the  index  indicate  the  voltage  to  which  the 
current  is  due. 

Describe  an  Ammeter. 

An  ammeter  is  identical  with  a  voltmeter,  except 
that  it  is  wound  with  thicker  wire,  and  of  fewer 
turns.  The  difference  between  them  is  this.  The 
ammeter  has  to  measure  the  entire  current  that  is 
flowing,  and  all  of  that  current  must  go  through  its 
coils.  The  voltmeter,  on  the  other  hand,  must  meas- 
ure an  exceedingly  small  fraction  of  the  current,  so 
small  that  it  can  be  taken  out  of  the  circuit,  without 
affecting  it.  Hence  an  ammeter  must  be  of  very  low 


86  QUESTIONS    AND    ANSWERS    ABOUT.  ELECTRICITY. 


WHEATSTONE'S    BRIDGE. 


ELECTRICAL   MEASUREMENT.  87 

resistance,    while  the   voltmeter   should  be   of   very 

high  resistance. 

Describe  a  Wheats  toners  Uridge. 

A  Wheatstone  bridge  is  a  contrivance  for  measuring 
the  resistance  of  any  conductor.  Imagine  two  con- 
ductors lying  side  by  side  in  a  straight  line,  with  their 
ends  connected  together  in  pairs,  and  a  current  sent 
through  them.  If  a  wire  is  now  connected  across 
them  and  is  so  placed  that  the  proportionate  resist- 
ances of  the  two  parts  of  one  conductor  shall  be  the 
same  as  that  of  the  two  parts  of  the  other  conductor, 
no  current  will  go  through  the  cross  connecting  wire. 
A  Wheatstone  bridge  consists  of  the  above  system  of 
connections,  with  a  very  sensitive  galvanometer  in 
the  cross  circuit.  The  four  divisions,  determined  by 
the  cross  circuit  connections  are  called  the  arms  of 
the  bridge.  The  conductor  whose  resistance  is  to  be 
measured,  forms  one  arm.  The  other  arms  are  made 
up  of  resistances,  all  of  which  are  known,  or  one  of 
which  is  known,  and  the  proportions  of  the  other  two 
to  each  other  are  known.  By  varying  these  resist- 
ances until  the  galvanometer  shows  no  current  the 
proportion  of  all  four  arms  to  each  other  is  known, 
and  hence  the  actual  resistance  of  the  unknown 
wire. 


QUESTIONS    AND    ANSWERS    ABOUT   ELECTRICITY. 


CHAPTER  VIII. 

GLOSSARY    OF    ELECTRICAL    TERMS. 

ACCUMULATOR.— See  battery  and  condenser. 

AMMETER. — An  instrument  for  measuring  current 
strength. 

AMPERE. — The  unit  of  current  strength.  It  is  the 
flow  of  electricity  produced  by  the  pressure  of  one 
volt  on  a  resistance  of  one  ohm. 

ARC. — The  stream  of  hot  gasses  and  particles  of 
matter  arising  from  the  boiling  or  vaporization  of 
two  conductors  when  a  current  is  passing  between 
them. 

ARMATURE. — That  part  of  a  dynamo  in  which  the 
current  is  induced.  It  may  be  a  stationery  or  mov- 
ing part,  but  is  generally  the  latter,  and  is  composed 
of  coils  of  wire  which  "cut"  the  lines  of  magnetic 
force  produced  by  the  fields.  This  "cutting57 
induces  a  current  in  the  coils. 

BATTERY. — One  or  more  cells  in  which  electricity 
is  produced  by  chemical  action.  There  are  two 
elements  of  different  substances  and  a  liquid  in 
every  voltaic  battery.  A  primary  battery  is  one 
in  which  the  "elements"  are  placed  and  used  until 
they  are  worn  out.  In  a  secondary  or  storage  bat- 
tery or  accumulator  the  "  elements  "  are  placed  in 
the  cell  and  first  " formed"  by  the  passage  of  a 


GLOSSARY  OF   ELECTRICAL   TERMS  89 

current  of  electricity  through  them.  The  cell  is 
then  said  to  be  charged  and  may  be  used  to  supply 
electricity.  The  term  battery  is  also  used  to  desig- 
nate a  collection  of  Leyden  jars  in  which  the  static 
electricity  is  stored. 

BRUSH. — A  collection  of  metal  sheets  or  wires  which 
press  against  the  commutator  of  a  dynamo  to  col- 
lect the  electricity,  or  of  a  motor  to  supply  it.  Car- 
bon brushes  are  coming  into  use  now,  especially  in 
railway  work. 

B.  &  S. — Brown  &  Sharp.  The  wire  gauge  used  in 
Amereica. 

B.  W.  G. — Birmingham   wire   gauge.      The   English 
wire  gauge. 

CELL. — The  jar  in  which  the  elements  and  liquid  of 
a  battery  are  placed.  The  term  is  used  also  for  the 
jar  and  its  contents. 

C.  G.  S. — The  abbreviation  of   centimeter,  gramme, 
second,  and  used  to  designate  the  so-called  absolute 
system  of  measurements. 

CIRCUIT. — A  system  of  conductors  over  which  elec- 
tricity passes. 

COIL  CLOSED. — The  coils  of  an  armature  are  said 
to  be  closed  when  the  end  of  one  is  connected  to 
the  beginning  of  the  next  at  the  commutator  bar. 
An  open  coil  armature  is  one  in  which  each  coil  is 
independent  of  the  others  and  has  its  own  commu- 
tator bars. 

COMMUTATOR.— That  part  of  a  dynamo  on  which 
the  current  from  the  armature  is  rectified  before 
passing  to  the  external  circuit.  The  current  in  a 


90          QUESTIONS   AND   ANSWERS    ABOUT   ELECTRICITY. 

given  section  of  an  armature  alternates  and  must  be 
made  continuous  on  leaving  it.  This  is  done  by 
the  commutator,  which  consists  of  a  series  of  insu- 
lated metal  bars  connected  to  the  armature  wires, 
and  so  placed  as  to  feed  into  different  brushes  as 
the  current  changes. 

CONDENSER. — An  apparatus  for  collecting  and 
holding  electricity.  It  consists  of  alternate  layers 
of  conducting  sheets  and  insulating  material,  the 
conductors  being  very  close  together,  and  the  ad- 
jacent ones  being  charged  with  the  opposite  kinds 
of  electricity.  Their  proximity  enables  them  to 
hold  a  larger  amount  of  electricity  than  they  could 
if  alone.  Condensers  are  sbmetimes  called  accumu- 
lators. 

CONDUCTOR.— A  substance  which  will  allow  the 
passage  of  electricity  over  it.  All  substances  will 
do  this,  but  some  to  so  small  an  extent  that  they 
are  called  insulators. 

COULOMB. — The  unit  of  electric  quantity.  It  is  the 
amount  of  electricity  which  flows  past  a  given  point 
in  one  second  on  a  circuit  conveying  one  ampere. 

CURRENT. — The  flow  of  electricity  in  a  conductor 
analogous  to  the  flow  of  water  in  a  pipe.  A  contin- 
uous current  is  one  that  does  not  change  its  direc- 
tion, while  an  alternating  current  is  one  that 
periodically  reverses. 

CUT  OUT. — An  arrangement  for  interrupting  a  cur- 
rent or  for  shunting  it  around  some  part  of  a 
circuit. 


GLOSSARY  OF   ELECTRICAL   TERMS.  91 

DYNAMO. — A  machine  driven  by  power  which  fur- 
nishes electricity. 

DYNAMOMETER. — An  apparatus  for  measuring 
the  power  given  out  or  consumed  by  a  machine. 
An  electro-dynamometer  is  an  instrument  for 
measuring  a  current  by  the  mutual  action  of  two 
coils  through  which  it  passes. 

E.  M.  F. — An  abbreviation  for  electro-motive  force. 
This  is  the  pressure  which  forces  the  electric  cur- 
rent through  a  conductor. 

ELECTRO-MAGNET.— A  magnet  produced  by  pass- 
ing a  current  through  a  coil  of  wire  around  a  soft 
iron  core.  The  core  is  magnetized  while  the  cur- 
rent flows,  but  loses  its  magnetism  when  the  current 
stops.  This  form  of  magnet  may  be  made  much 
more  powerful  than  a  permanent  magnet,  and  is 
therefore  used  in  place  of  the  latter  in  dynamos. 

FARAD. — The  unit  of  capacity.  A  condenser  that 
will  hold  one  coulomb  at  a  pressure  of  one  volt  has 
a  capacity  of  one  farad. 

FILAMENT. — In  an  incandescent  lamp  the  thread  of 
carbon  which  becomes  luminous  when  the  current 
is  passed  through  it. 

GALVANOMETER.— An  instrument  for  detecting 
and  measuring  the  electric  current  by  the  action  of 
a  coil  of  wire  upon  a  magnetic  needle. 

INDUCTION. — A  current  is  said  to  be  induced  in  a 
conductor  when  it  is  caused  by  the  conductor  cut- 
ting lines  of  magnetic  force.  A  fluctuating  current 
in  a  conductor  will  tend  to  induce  a  fluctuating  cur- 
rent in  another  running  parallel  to  it.  A  static 


92          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

charge  of  electricity  is  induced  in  neighboring  bodies 
by  the  presence  of  an  electrified  body.  A  magnet 
"induces"  magnetism  in  neighboring  magnetic 
bodies. 

INDUCTION  COIL.— An  arrangement  by  which  an 
alternating  or  fluctuating  current  in  a  coil  of  wire 
will  induce  an  alternating  current  in  a  parallel  coil. 

INSULATOR.— The  opposite  of  a  conductor.  A 
body  which  will  not  allow  the  passage  of  electricity 
except  in  such  small  quantities  as  to  be  negligable. 

LINES  OF  FORCE.— Imaginary  lines  which  radiate 
from  a  magnet  and "  show  by  their  direction  the 
path  which  a  free  magnetic  pole  would  take  if  left 
to  itself.  Conventionally,  the  strength  of  a  mag- 
netic field  is  indicated  by  the  number  of  these  lines. 
Their  form  is  shown  by  the  well-known  experiment 
with  the  magnet  and  iron  filings. 

MAGNET. — A  body  possessing  the  property  of  at- 
tracting iron,  steel  and  a  few  other  metals. 

MAGNETIC  FIELD.— The  space  around  a  magnet 
in  which  its  power  of  attraction  is  exhibited. 

MULTIPLE  or  MULTIPLE  ARC.— A  method  of 
connecting  electric  conductors  by  which  a  number 
of  sources  of  electricity  feed  directly  into  or  a  unm- 
ber  of  receivers  of  electricity  take  it  directly  from 
the  same  mains. 

NEGATIVE. — A  conventional  term  to  indicate  the 
direction  of  flow  of  a  current,  or  the  state  of  elec- 
trification of  a  body.  The  negative  or  terminal  of 
a  dynamo  is  the  one  at  which  electricity  enters  it 
from  the  external  circuit,  while  the  negative  ter- 


GLOSSARY   OF   ELECTRICAL   TERMS.  93 

minal  of  a  lamp  or  instrument  is  that  connected  to- 
wards the  negative  terminal  of  a  dynamo.  It  is 
designated  by  — 

OHM — The  unit  of  electrical  resistance. 

OHMS  LAW. — States  that  the  current  in  any  circuit 
is  equal  to  the  E.  M.  F.  acting  on  it  divided  by  its 
resistance. 

PERMANENT  MAGNET.— A  piece  of  hardened 
steel  which  retains  its  magnetism  after  the  magnet- 
izing influence  is  removed. 

PARALLEL.— See  Multiple. 

POLE. — Those  parts  of  a  magnet  which  show  the 
strongest  magnetic  force.  In  a  bar  magnet  this  is 
generally  a  short  distance  from  the  ends.  The  pole 
of  a  dynamo  or  battery  is  one  of  its  terminals. 

POSITIVE. — A  conventional  term  to  show  the  direc- 
tion of  a  current.  In  a  dynamo  or  battery  it  is  the 
terminal  at  which  the  electricity  leaves  it.  It  is 
designated  by  + . 

RESISTANCE.— The  opposition  offered  by  a  body 
to  the  passage  of  electricity  through  it. 

RHEOSTAT. — An  apparatus  for  throwing  a  variable 
resistance  into  a  circuit  at  will. 

SERIES. — Two  or  more  conductors  are  said  to  be  in 
series  when  they  are  so  connected  that  the  same 
current  that  passes  through  one  passes  through  the 
other. 

SHORT  CIRCUIT.— An  indefinite  term  used  gen- 
erally in  the  case  of  dynamos  and  batteries  for  a 
resistance  between  the  terminals  lower  than  the 
machine  or  battery  is  calculated  to  stand  or  run  on 


94          QUESTIONS   AND   ANSWERS   ABOUT   ELECTRICITY. 

in  practice.  With  lamps  the  term  is  used  for  a 
low  resistance  between  the  terminals,  which  de- 
prives it  of  most  of  the  current. 

SHUNT. — A  shunt  is  a  conductor  connected  around 
another  in  such  a  way  that  it  deprives  the  first  of  a 
part  of  the  current. 

SOLENOID.— A  hollow  coil  of  wire. 

VOLT. — The  unit  of  electro-motive  force  or  pressure 
analogous  to  the  head  of  water  in  hydraulics. 

VOLTMETER.— An  instrument  for  measuring  the 
voltage  or  pressure  on  a  circuit. 

WATT.— The  unit  of  work.  The  watts  developed 
in  a  circuit  are  equal  to  the  current  multiplied  by 
the  E.  M.  F.  Seven  hundred  and  forty-six  watts 
equal  one  horse  power. 

WATTMETER. — An  instrument  for  measuring  the 
electrical  energy  in  a  circuit. 


I  N  DKX. 


Accumulator 33,  37 

Arc,  electric 58.  60 

lamp 60,  62 

Armature,  what  is  an 47 

drum 48 

difference  between  ring  and  drum 52,  53 

disc 49 

pole 49 

Ammeter,  description  of 85 

Ampere 78 

Alphabet,  Morse  telegraph 69 

Amalgamated  zinc 40 

Apparatus,  miscellaneous 66,  76 

B 

Batteries,  different  kinds  of 33 

Battery,  voltaic 31 

accumulator 33,  36 

Bunsen 32,  33 

Grove 32,  33 

Grenet 33,  34 

gravity 33,  34 

Leclanche 33,  35 

storage 33,  37 

Daniels 33 

Samson 33,  36 

connected  for  different  purposes 37,  38 

chemical  action  in  a 39 

local  action  in  a 40 

polarization  of  a 41,  42 

Brushes,  their  use  upon  dynamos  and  motors 52 

Bell,  electric 66 

Bodies,  magnetic 27 

diamagnetic 27 


96  INDEX. 


Currents,  electric 11,  14 

how  measured 77 

Circuit,  resistance  in  a 80 

Conductor 15 

non 16 

list  of 16 

Coulomb 79,  80 

Condenser 22 

Connecting  in  series 38 

in  multiple 38 

in  multiple  series 38 

in  series  multiple 39 

batteries  for  different  purposes 37 

Coil  induction 73 

Kuhmkorff 74 

Cell,  accumulator 33,  37 

storage 33,  37 

Bunsen 32,  33 

Grove 32,  33 

gravity 33,  34 

Grenet 33,  34 

Leclanche 33,  35 

Daniels     33 

Samson 33,  36 

voltaic 31 

connected  for  different  purposes 37,  38 

chemical  action  of  a 39 

local  action  of  a 40 

polarization  of  a , . . .. 41 

Commutator 51 

its  use 51 

Collector  ring 53 

D 

Dynamo,  alternating  current 45 

direct  current 44,  45 

description  of  a 43 

collector  ring  upon 53 

brushes  upon 52 

difference  between    series,    shunt  and   com- 
pound wound 54,  57 

dynamic  electricity 14 

diamagnetic  bodies 27 


97 


Earth  magnetism 26 

Electricity,  atmospheric 12 

frictional 11 

dynamic 14 

magneto 14 

pyro 13 

static 11 

thermo 12 

word  derived  from 10 

Electric  arc 58 

lamps 60,  65 

miscellaneous  apparatus 66,  76 

bell 66 

telegraph 66,  71 

telephone 71 

welding 76 

Electrical  attraction 10 

measurement 77,  87 

repulsion 10 

units 78,  81 

Electroscope 15 

Energy,  unit  of : 80 

F 

Farad 78 

Field,  magnetic 30 

magnets 49 

Force,  magnetic / 27 

laws  of  magnetic 27 

G 

Gravity  cell 33,  34 

Grenet  cell 33,  34 

Grove  cell 32,  33 

Galvanometer,  construction  of 81,  82 

H 

Holtz  machine 17 

I 

Induction 11 

coil 73 

Incandescent  lamp 62,  65 

Influence  machines 16,  24 


98  INDEX. 

J 

Jar,  Leydeu 23,  24 

L 

Lamp,  arc 60,  62 

incandescent 62,  65 

Law  of  inverse  squares 79 

of  magnetic  force 27 

Ohms 80 

Leyden  jar 23,  24 

M 

Machine,  influence 16,  24 

dynamo 43,  46 

magneto .    49 

welding 76 

Holtz 17 

Topler 19 

Magnet,  artificial 28 

electro 28 

permanent 28 

strength  of 29 

the  two  poles  inseperable 29 

field 49 

natural 27 

act  across  bodies 26 

loss  of  strength 29 

Magneto  machine 49 

Magnetic  attraction 25,  26 

pole 29 

saturation 29 

repulsion 25,  26 

bodies 27 

force    27 

field 30 

needle 30 

Magnetism  derived  from  the  earth 26 

theory  of 25 

residual 29 

Multiple,  connecting  in 38 

series 38 

Microphone 71,  72 

Measurement,  electrical 77,  87 

Motors,  electric 47 


INDEX.  99 

N 

Needle,  magnetic  30 

o 

Ohms  law 80 

P 

Pole,  magnetic 29 

Polarization,  remedies,  etc 41 

Potential 81 

Pressure,  unit  of 80 

R 

Resistance  in  a  circuit 80 

Ruhmkorff  coil 74 

Repulsion,  electrical 10 

Relay 69 

S 

Squares,  law  of  inverse 79 

Storage  cell 33,  37 

Strength  of  a  magnet 29 

loss  of 29 

unit  of  current 81 

Saturation,  magnetic 29 

Series,  connecting  in 38 

multiple.  1 38 

Shunt-wound  dynamo 56 

Static  electricity 11 

T 

Theory  of  electricity 9 

of  magnetism 25 

Transformer 74,  75 

Telephone 71 

Telegraph 66,  69 

alphabet 69 

diagram  of  circuit 70 

u 

Units,  names  of  electrical 78,  79 

of  quantity 79 

of  pressure 80 

of  current  strength 81 

of  rate  of  electric  work 81 


100  INDEX. 

V 

Volt 80 

Voltmeter 85 

W 

Watt 81 

Wheatstone's  bridge 86,  87 

Winding  of  dynamos 57 

Welding,  electric 76 

Z 
Zinc,  amalgamation  of 40 


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The  Electro-Magnet   and  Electro-Magnetic  Mechanism. 

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Tunzelmann,  G.  W.  de.  Electricity  in  Modern  Life.  Illus- 
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Walker,  Sydney  F.  Electricity  in  our  Homes  and  Workshops. 
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Webb,  H.  TJ.  A  Practical  Guide  to  the  Testing  of  Insulated 
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Wormell,  K.  Electricity  in  the  Service  of  Man.  A  Popular 
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"  EVERYBODY'S  HAND-BOOK  OF  ELECTRICITY,"  by  Edward 
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descent Wiring,  120  Pages,  50  Illustrations.  Giving  a  descrip- 
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"How  TO  MAKE  ELECTRIC  BATTERIES  AT  HOME,"  by  Ed- 
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mation needed  to  make  simple,  yet  practical,  electrical  batter- 
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"EXPERIMENTAL  ELECTRICITY,"  by  Edw.  Trevert.  176  Pages, 
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meters, electric  motors,  dynamos,  magnetos,  etc. 

Price,  $1.00,  Cloth  Bound. 


"DYNAMOS  AND  ELECTRIC  MOTORS,"  AND  ALL  ABOUT  THEM, 
by  Edw.  Trevert.  Fully  Illustrated.  This  book  gives  complete 
directions  for  making  Dynamos  and  Motors;  also  tells  you  all 
about  them.  Price,  50  cts.,  Cloth  Bound. 


"ARMATURE     AND     FlELD-MAGNET     WINDING,"     by  Edward 

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practical  character  and  the  formulae  use  is  of  the  highest 
authority,  by  which  one  can  wind  a  dynamo  or  motor  for  any 
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"How  TO  MAKE  A  DYNAMO,"  by  Edward  Trevert.  Illustra- 
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Price,  50  cts.,  Paper.. 


"ELECTRIC  RAILWAY  ENGINEERING,"  by  Edward  Trevert., 
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"  A  PRACTICAL  TREATISE  ON  ELECTRO-PLATING,"  by  Edward' 
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ELECTRICITY  AND  ITS  RECENT  APPLICATIONS,"  by  Edward 
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and  most  complete  book  of  the  present  time,  and  is  particularly 
adapted  to  the  use  of  students.  Contains  20  chapters  of  the 
best  and  latest  experiments,  also  complete  working  directions 
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Price,  $2.00,  Cloth  Bound- 


"  A  PRACTICAL  TREATISE  ON  THE  INCANDESCENT  LAMP,"  by 
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the  Thomson-Houston  Co.  Illustrated.  This  is  the  only  work 
that  explains  in  a  practical  manner  the  manufacture  of  the 
Incandescent  Lamp,  and  should  be  owned  by  every  Electrician 
and  Student  interested  in  the  subject. 

Price,  50  cts.,  Cloth  Bound. 


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anybody  wishing  to  construct  their  own  electrical  apparatus. 
Giving  complete  directions  and  working  drawings  for  making 
an  electric  motor  for  running  sewing  machines,  small  lathes, 
etc.  Also  gives  directions  and  drawings  for  building  an  elec- 
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Price,  $1.00,  Cloth. 

"A  HAND-BOOK  OP  WIRING  TABLES,"  for  Arc,  Incandescent 
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up  to  date.  Price,  75  cts.,  Cloth. 


"How  TO  MAKI;  AND  USE  INDUCTION  COILS,"  by  Edward 
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"  QUESTIONS  AND  ANSWERS  ABOUT  ELECTRICITY."  A  first 
book  for  students.  Theory  of  Electricity  and  Magnetism. 
Edited  by  E.T.  Bubier,2d.  Authors:  T.  O'Connor  Sloane,  A.  M., 
E.  M.,  PH.  D.,;  Caryl  D  Haskins,  M.  I.  E.  E. ;  A.  E.  Watson; 
Edward  Trevert.  Illustrated.  Contents:  Chapter  1. — Theory 
of  Electricity.  II  — Theory  of  Magnetism.  III. —Voltaic 
Batteries.  IV.  —Dynamos  and  Motors.  V.— Electric  Lamps. 
VI.  —  Miscellaneous  Electrical  Apparatus.  VII. — Electrical 
Measurement.  Price,  Cloth  Bound,  50  cents. 


"  TRANSFORMERS."  Their  Theory,  Construction  and  Applic- 
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book  for  the  use  of  everybody.  Fully  illustrated. 

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General  Library 

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GENERAL  LIBRARY  -  U.C.  BERKELEY 


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